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169

Geological Society of AmericaField Guide 8

2006

Rivers, glaciers, landscape evolution, and active tectonics of the central Appalachians, Pennsylvania and Maryland

Frank J. PazzagliaEarth & Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA

Duane D. BraunGeography & Geosciences, Bloomsburg University, Bloomsburg, Pennsylvania 17815, USA

Milan PavichEastern Earth Surface Processes Team, U.S. Geological Survey, Reston, Virginia 20192, USA

Paul BiermanDepartment of Geosciences, University of Vermont, Burlington, Vermont 05405, USA

Noel Potter, Jr.Department of Geology, Dickinson College, Carlisle, Pennsylvania 17013, USA

Dorothy MerrittsRobert Walter

Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA

Dru GermanoskiDepartment of Geosciences, Lafayette College, Easton, Pennsylvania 18042, USA

INTRODUCTION

Welcome to the Appalachian landscape! Our fi eld trip begins with a journey across Fall Zone (Fig. 1), named for the falls and rapids on streams fl owing from the consolidated rocks of the Appalachians onto the unconsolidated sediments of the Coastal Plain. The eastern U.S. urban centers are aligned along the Fall Zone, the upstream limit of navigation. Typically, the rocks west of the Fall Zone are part of the Piedmont province. This province exposes the metamorphic core of the Appala-chian Mountains exhumed by both tectonics and erosion. At least four major phases of deformation are preserved in Piedmont rocks, three Paleozoic convergent events that closed Iapetus, followed by Mesozoic extension that opened the Atlantic Ocean. A record of Cretaceous to Quaternary exhumation of the Appalachians is preserved as Coastal Plain sediments. Late Triassic and Jurassic erosion is preserved in the syn-extensional fault basins, such as the Newark basin, or is buried beneath Coastal Plain

Pazzaglia, F.J., Braun, D.D., Pavich, M., Bierman, P., Potter, N., Jr., Merritts, D., Walter, R., and Germanoski, D., 2006, Rivers, glaciers, landscape evolution, and active tectonics of the central Appalachians, Pennsylvania and Maryland, in Pazzaglia, F.J., ed., Excursions in Geology and History: Field Trips in the Middle Atlantic States: Geological Society of America Field Guide 8, p. 169–197, doi: 10.1130/2006.fl d008(09). For permission to copy, contact [email protected]. ©2006 Geological Society of America. All rights reserved.

170 Pazzaglia et al.

DAY 1. TRIP FROM GREAT FALLS, VIRGINIA, TO NORTH EAST, MARYLAND, ALONG THE MIDDLE ATLANTIC FALL ZONE

Exit the Comfort Inn Ballston and proceed north on Rt. 120 to I-66 W, to Rt. 267 W, to I-495N, cross the Potomac River, to Exit 40, Rt. 190 W toward the town of Potomac. At Potomac, turn left on Falls Road and proceed south to Great Falls National Park.

STOP 1.1. Great Falls of the Potomac

The purpose of this stop is to view a knickpoint in the channel of the Potomac River and compare and/or contrast it with the Holt-wood Gorge area of the Susquehanna River which we will visit on Day 2. The results reported here are summarized from Bierman et al. (2004). Great Falls is a knickpoint initiated probably by one or more Pleistocene eustatic drawdowns. The upstream lip of the knickpoint is analogous to the island tops at Holtwood. The long-term incision history of the Potomac River is constrained by strath terraces, preserved here at Great Falls, down stream in the Mather Gorge, and upstream in the adjacent uplands (Zen, 1997a, 1997b). The adjacent uplands on the Virginia side of the river are mantled

with a late Miocene or Pliocene upland gravel (Fleming et al., 1994). Eighteen samples for cosmogenic 10Be and 26Al analysis were collected from exposed, fl uvially eroded outcrops of quartz-bearing schist along strath terraces in Mather Gorge below Great Falls (Bierman et al., 2002; Reusser et al., 2004; Fig. 2). The most prominent strath terrace, a several-km-long bedrock feature 20–25 m above the current low water level, was apparently aban-doned rapidly as the Potomac River incised ca. 30 ka. Nine sam-ples, collected from water-polished rock surfaces down a cross section from this terrace to just above the river, have decreasing nuclide activities consistent with a fl uvial, bedrock incision rate of ~70 cm/k.y. (700 m/m.y.) and an effective 6 k.y. exposure age at water level (corrected for cosmic-ray dosing at and just below the water’s surface). The implications of these results are: (1) The most distinct bedrock strath terrace bordering the Potomac River downstream of Great Falls is a time-transgressive feature that is ca. 38 ka at Bear Island, but considerably older downstream in the Mather Gorge; (2) strath formation and retreat of the knickpoint face has been unsteady; (3) the Great Falls knickpoint fi rst formed between 25 and 30 ka; and (4) the average rate of incision of the Mather Gorge has been between 500 and 800 m/m.y. over much of the late Pleistocene-Holocene.

sediments (Fig. 1). The trip proceeds northwest across the Fall Zone and Piedmont and into the Newark basin. Late Triassic and Jurassic fl uvial red sandstone, lacus-trine gray shale, and black basalt were deposited in this basin. The Newark basin is separated from the Blue Ridge by a down to the east normal fault that locally has con-temporary microseismicity. The Blue Ridge represents a great thrust sheet that was emplaced from the southeast during the Alleghenian orogeny (Permian). The sum-mits of the Blue Ridge are commonly broad and accordant. Davis (1889) projected that accordance westward to the summits of the Ridge and Valley to defi ne his highest and oldest peneplain—the Schooley peneplain. North and west of the Blue Ridge is the Great Valley Section of the Ridge and Valley Province (Fig. 1). Where we cross the Great Valley at Harrisburg, it is called the Cumberland and Lebanon valleys. This section is underlain by lower Paleozoic carbonate, shale, and slate folded and faulted during the lower Paleozoic Taconic orogeny. The prominent ridge on the west fl ank of the Great Valley is Blue or Kittatinny Ridge. It is the fi rst ridge of the Ridge and Valley Province; the folded and faulted sedimentary rocks of the Appalachian foreland basin, deformed during the Alleghenian orogeny. Drainage during most of the Paleozoic was to the northwest, bringing detritus into the Appalachian foreland basin. The drainage reversed with the opening of the Atlantic Ocean and southeast-fl owing streams established courses transverse to the strike of resistant rocks, like the Silurian Tuscarora Sandstone holding up Blue Mountain. West and north of the Ridge and Valley is the Allegheny Plateau, that part of the Appalachian foreland that was only gently deformed during Alleghenian shortening. Our trip will traverse that part of the plateau called the Pocono Plateau which is underlain by Devonian to Penn-sylvanian sandstone. At the conclusion of our trip, we will reverse our transverse of the Appalachians by traveling from the Pocono Plateau to the Ridge and Valley, to the Great Valley, to the Newark Basin, to the Piedmont, and then to one of the great Fall Zone cities—Philadelphia—via the Lehigh and Schuylkill rivers.

Keywords: Appalachian geomorphology, erosion rates, landscape evolution, glacial geology.

Rivers, glaciers, landscape evolution, and active tectonics 171

Return to I-495 (Capitol Beltway) and proceed north to Rt. 295 (Baltimore-Washington Parkway). Continue north on Rt. 295 toward Baltimore, to I-895 and the Harbor Tunnel. Continue to the end of I-895 where it merges with I-95 north. Take I-95 north for 30 mi to the Susquehanna River, cross the river, and continue for 10 more mi to exit 100, North East, Maryland. Take Rt. 272 south, toward the town of North East. Turn right at the intersec-tion with Rt. 40 and proceed for ~5 mi to Belvedere Road. Turn Right on Belvedere Road and proceed north to the entrance to the York Quarry Belvedere plant on the left.

STOP 1.2. Bryn Mawr Formation at the Belvedere Pit

The purpose of this stop is to build an understanding of the Coastal Plain stratigraphy at the head of Chesapeake Bay and begin relating that stratigraphy to a series of upland gravels that onlap the Fall Zone (Figs. 3 and 4). The gravels exposed here are the down-dip Fall Zone equivalent of the Piedmont upland gravels and correlate basinward to biostratigraphically dated marine Coastal Plain deposits. The large gravel pits here are oper-ated by York Building Products and they quarry a quartzose sand

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Figure 1. Digital relief map showing the trip route and stops. Numbered circles denote stops. First number is day and second number is stop. Circled numbers are major roads and interstates. Physiographic provinces: AP—Appalachian Plateau, BR—Blue Ridge, CP—Coastal Plain, FZ—Fall Zone, GV—Great Valley, NB—Newark Basin, P—Piedmont, PP—Pocono Plateau, RV—Ridge and Valley. Glacial limits (dashed lines): W—late Wisconsinan, PI—pre-Illinoian.


and gravel called the Bryn Mawr Formation (Pazzaglia, 1993). It is just one of several upland gravel deposits mapped at the head of Chesapeake Bay. The Bryn Mawr Formation unconformably overlies the Cretaceous Potomac Group as well as a well devel-oped saprolite in the underlying bedrock. The sand and gravel is both texturally and compositionally mature. The light fraction is virtually all quartz, and the heavy minerals are zircon, tourmaline, rutile, ilmenite, and leucoxene. There are several lithofacies pres-ent in the highwalls at any time during the quarrying operations. The dominant facies is a large-channel fl uvial facies character-ized by large upper delta plain lateral accretion surfaces draped by laminated clays. Subordinate facies include channel thalweg gravels, oxbow lakes (pollen bearing), middle delta plain shore-line, and lower delta plain distributary mouth bars. In 1994, the quarry operators broke into and collected a deposit of black clay. That clay is very rich in plant remains and pollen. An analysis of the pollen reveals that it is a late middle Miocene (ca. 12 Ma) assemblage (Pazzaglia et al., 1997). Bryn Mawr Formation–like gravels occur all along the mid-Atlantic Fall Zone where they are called the Bon Air Formation (Virginia) Brandywine Formation (Maryland), and Beacon Hill Formation (New Jersey). It is very likely that such widespread gravel deposition occurred during middle and late Miocene eustatic highstands, at a time when the Appalachians were shedding a deeply weathered regolith. Up-dip, the Bryn Mawr gravels are represented by at least three river terraces. Down dip, they are generally equivalent to the Choptank

Formation, and to a lesser degree, the Calvert and St. Mary’s For-mations (Fig. 4).

Retrace route south on Belvedere Road to Rt. 40. Turn left and proceed to North East, Maryland. Turn right onto Rt. 272 and continue to the terminus of the road at the parking area for the Turkey Point lighthouse.

STOP 1.3. Stratigraphy of the Pensauken Formation and Turkey Point beds at Turkey Point

The purpose of this stop is to observe a younger group of Coastal Plain upland gravels and discuss their genesis in the con-text of changing base level along the inner margin of the Fall Zone. The sea cliffs at Turkey Point are underlain mostly by the red, arkosic Pensauken Formation (Campbell and Bascom, 1933). This fl uvial to fl uvial-estuarine deposit unconformably overlies the Cretaceous Potomac Group and is unconformably overlain by a newly defi ned, but unoffi cial unit called the Turkey Point beds (Fig. 5). Despite its location at the mouth of the Susquehanna River, the Pensauken Formation is an ancestral Delaware-Hud-son River deposit. Like all other central Appalachian streams, the Hudson River formerly made a sharp bend to the southwest at the Fall Zone. Beginning in the late Miocene, the paleo-Hudson swung across southern New Jersey depositing a subarkosic sand-stone called the Bridgeton Formation with down-dip equivalents

Figure 2. Composite record of strath elevations along the Potomac River gorge complex, Great Falls to tidewater. The ki-lometer scale is measured from Gladys Island; the miles scale is measured from Chain Bridge, as originally used by Reed and others, (1980). Vertical uncertainty refers to the uncertainty in location of individual outcrops during mapping. (A) Data: circles—rock summits; crosses—channels and scour ponds; up-pointing triangles—rock benches; down-pointing tri-angles—outlets of plunge pools; squares—lateral potholes. Profi le of channel fl oor based on sounding by J.C. Reed (1983, written commun.). (B) Interpretation of data as strath levels (Zen, 1997a), not including Glade Hill and Matildaville straths that are higher than the Bear Island level. BI—Bear Island, SL—Sandy Landing, BP—Black Pond, HG—Hidden Gorge, PC—Plummers Channel (from Bierman et al., 2004).


Figure 3. Map (above) and conceptual cross section (below) of the fl uvial deposits on the Fall Zone and upper Coastal Plain at the head of Chesapeake Bay (from Pazzaglia, 1993).


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beneath the Delmarva Peninsula called the Bethany and Manokin formations (Pazzaglia, 1993). Inset into this late Miocene fl uvial deposit is the Pliocene (age palynologically confi rmed) Pensauken Formation. It covers the upper two thirds of Delmarva (where it is called the Columbia Formation) and is equivalent down dip to the Beaverdam and Yorktown formations (Pazzaglia, 1993; Fig. 4). Here at Turkey Point it is apparent that the Pensauken Formation is predominantly fl uvial in origin, with minor silty clay beds, perhaps

indicative of local limited estuarine conditions. Reactivation sur-faces between waxing/waning packages of 12–14 foresets indi-cates local tidal conditions (Duke, 1991, personal commun.). The Pensauken Formation is unconformably overlain by a fi ning-up sequence of white to buff fl uvial gravel to estuarine mud called the Turkey Point beds. These deposits have a petrography consistent with a Susquehanna River source. Turkey Point is one of the few places where the coarse grained bouldery facies is present; else-where, the mud-rich facies, which is locally laminated, prevails. The muddy facies is interpreted as an ancestral, maybe early Pleis-tocene, Chesapeake Bay. Clearly, the weathering profi le formed at the top of the Turkey Point beds shows at least three buried pro-fi les, the fi rst two of which are developed in loess (Woodfordian and lllinoian?). Magnetostratigraphy through the soils and into the estuarine deposits remains normal until a subtle unconformity is crossed whereupon reversed polarity is found. If this reversal is taken as the Brunes-Matuyama boundary, the Turkey Point beds would be early Pleistocene in age.

Retrace route out to North East, Maryland, along Rt. 272 and continue to the intersection of Rt. 272 and I-95 to the Crystal Inn hotel. Accommodations in the evening at the Crystal Inn and dinner in North East.

DAY 2. THE FALL ZONE, THROUGH THE PENNSYLVANIA PIEDMONT TO THE HOLTWOOD GORGE, ENDING IN THE GREAT VALLEY ALONG THE NORTHWEST FLANK OF THE BLUE RIDGE

Exit hotel to I-95 south, continue to exit 93 and exit for Rt. 222 north. Follow 222 north to Port Deposit, then along the lower Susquehanna River to the Conowingo Dam. Stay to right along Rt. 222-Rt. 1, then turn left at the light, remaining on Rt. 222. After ~3 mi, turn left onto Pilot Town Road and proceed to Pilot. Turn right at Pilot onto Pleasant Grove–Pilot Town Road and proceed to the Mason-Dixon line, then 2 more mi to Black Baron. Bear left onto Riverview Road, then stay right onto Cherry Hill Road. Pull off to side of road and park on the top of the hill.

STOP 2.1. Kirk Farm Gravels

The purpose of this stop is to visit an upland gravel locality to study its texture and composition, and suggest a correlation through the Piedmont into the Fall Zone. This is a “famous” location on the Francis Kirk farm where there are large quan-tities of indisputably fl uvial-origin “potato stones.” Locally, the fl uvial gravels are still in place or nearly in place, as they are cemented by hematite. The site is at an elevation of 500 ft (150 m) on the Pennsylvania Piedmont. Petrography reveals that they are virtually all vein quartz in composition and very likely proximal fl uvial equivalents to the Bryn Mawr Forma-tion. A pit dug at this location showed that the gravel is part of a matrix-supported colluvial deposit that is ubiquitous on the Pennsylvania Piedmont (Pazzaglia and Gardner, 1993). The Figure 5. Stratigraphic section at Turkey Point.


gravel is found throughout at least 2–3 m of colluvium that lies atop saprolite and bedrock. Presumably, the gravel was once part of a stratifi ed, more extensive deposit, but has been eroded by the intense periglacial activity that has repeatedly affected this region during in the middle and late Pleistocene. Because this gravel deposit is highest in the landscape, it is mapped as Tgl or the most ancient, recognizable terrace of the lower Susque-hanna River. The change in gravel composition with respect to age is very well expressed in the surrounding area as the gravels exposed at the Brinton Farm only two miles from here, but at an elevation of 340 ft (104 m), contain numerous sandstone, silt-stone, and chert not found at Kirk Farm. Petrography forms the basis for correlation of terraces through the Piedmont (Fig. 6; Pazzaglia and Gardner, 1993). Virtually every fl at fi eld in the Piedmont of York and Lancaster Counties has been walked and inspected. Fluvial gravel lags like the one exposed here are only found when the fi eld is (1) within 2 km of the river, and (2) at a specifi c elevation corresponding to the three main terraces (here at 500, 400, and 340 ft).

Continue north on Cherry Hill Road. At the next intersection, make a right turn onto Rigby Road, continue to the merge with Pilot Town Road, then proceed to Rt. 272. Turn left on Rt. 272 and proceed for ~8 mi to the blinking light at Buck. Turn left at the blinking light on to Rt. 372 and proceed west toward the Susquehanna River.

STOP 2.2. Norman Wood Bridge

The purpose of this stop is to observe the bedrock channel of the Susquehanna River and discuss the processes of strath gen-esis. Walk out on bridge to observe Susquehanna channel. The view from the bridge affords an outstanding perspective on the Holtwood Islands and the modern Susquehanna channel. At low water, this is an impressive active strath, and it indicates this riv-er’s ability to carve out a wide valley bottom while still vertically incising. The extreme eastern channel bank is not a fl at strath, but rather has been revealed as one of six “deeps” (Mathews, 1917) along the Piedmont reach of this river. Deeps are spoon-shaped, but not connected, excavations that tend to hug the eastern side of the modern channel. They are intensely potholed and sculpted. The deep here at Holtwood is over 40 m which places its base ~6 m below sea level. The deeps are superimposed on a lower Susquehanna long profi le that is convex from Harrisburg all the way to tidewater (Fig. 7).

The many islands that dot the Holtwood area attest to rela-tively rapid incision of a paleo-strath through this reach (Thomp-son, 1990). Note that even though the island tops appear fl at, a correlation based on the tops alone suggests an upstream dip (Fig. 8). The passage of a knickpoint at a rate similar to the verti-cal down wearing of the channel would result in a time-transgres-sive, upstream-dipping strath, here represented by the island tops, that is, become older downstream, but young upstream where it merges with the modern strath just in front of the dam.

Strath genesis is probably linked to times when the river has more abrasive tools, such as during glacial outwash. Wide straths are favored by both the availability of tools as well as protracted periods of base-level stability. In contrast, the strath is destroyed by vertical incision, concentrated in numerous rivulets, during times of low bedload fl ux and/or during times of base-level fall. Upstream migration of knickpoints, initiated at the Fall Zone by impulsive glacio-eustatic fall, is an additional mechanism for channel bed lowering. The strath exposed here, presumably carved during the late Pleistocene, is currently being destroyed by vertical incision of numerous small channels and rivulets. If the island tops are a paleostrath, it was abandoned by both verti-cal incision, as well as the passage of a knickpoint.

Continue west on PA Rt. 372, and cross the Norman Wood bridge. Turn right immediately after the bridge into the Lock 12 parking area.

STOP 2.3. Holtwood Islands

The purpose of this stop is to inspect the islands at Holtwood and discuss the mechanisms of bedrock channel erosion, includ-ing the formation of potholes and the deeps. The islands along the western channel bank are easily reached by following the blue blazes of the Mason-Dixon trail. Potholes are an important fea-ture on the island tops and along their sides. The potholes, along with the deeps, have been used to argue for large, perhaps cata-strophic fl oods in the lower Susquehanna River during the Pleis-tocene (Thompson, 1985, 1990) in part because these features scale with similar features associated with the megafl ood-derived channeled scabland fl oods of the Columbia River. Two poten-tial sources of catastrophic fl oods have been suggested. The fi rst has an origin along the North Branch of the river as catastrophic sub-glacial fl oods responsible for shaping the drumlin fi elds of New York State (Shaw, 1989). We will review the evidence on Day 3 for the unlikelihood of such fl oods actually coming down the North Branch of the river. The other possible origin of cata-strophic fl oods is the failure of an ice dam along the West Branch of the Susquehanna River and the rapid draining of glacial Lake Lesley. The presence of Lake Lesley has been recently verifi ed (Sasowsky, 1994; Gardner et al., 1994; Ramage et al., 1998) as an early Pleistocene, ice-dammed lake in the Bald Eagle valley of central Pennsylvania. Alternatively, the potholes and the deeps are not relict, but rather modern features actively being carved by the river. The presence of similar features on rivers such as the Potomac or tributaries to the Susquehanna River that never expe-rienced glaciation in their headwaters argue against catastrophic fl oods as the cause. Furthermore, the Susquehanna River has generated large historic discharges such as hurricane Agnes (>106 cfs), and spring rain-on-snow events have generated historic ice-choked discharges in excess of several hundred thousand cfs.

New cosmogenic dating of four strath surfaces here in the Holtwood Gorge provide the fi rst quantifi ed look at the rates of river incision for this reach of the Susquehanna (Reusser et al.,


west bank terraces

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Figure 6. Proposed correlation of upland gravels with respect to the Susquehanna River long profi le through the Pennsylvania Piedmont (From Pazzaglia and Gardner, 1993).

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Figure 7. Longitudinal profi le of the lower Susquehanna River illustrating the location of the deeps through the Piedmont reach.


2004, 2006; Fig. 9). Exposure ages modeled from 10Be activities indicate that fl uvially eroded bedrock surfaces within Holtwood gorge increase predictably in age with height above the channel fl oor, and that all are late Pleistocene features. The highest well-preserved terrace (level 3) yields a mean exposure age of 36.1 ± 7.3 ka (n = 14). The middle and lowest terraces, levels 2 and 1, yield mean exposure ages of 19.8 ± 2.7 ka (n = 20) and 14.4 ± 1.2 ka (n = 10), respectively. One-way ANOVA demonstrates that the terrace ages are distinguishable (p < 0.0005), confi rm-ing that the three levels do indeed represent separable periods of strath formation and terrace abandonment. Two samples collected from heavily weathered and eroded topographic high points (LR-01 and LR-43), standing >20 m above the channel fl oor, yield model ages of 97.1 ± 10.5 ka and 84.5 ± 9.1 ka, respectively. Because the bedrock sampled at these two locations was shat-tered and no longer preserved water-polished surfaces, we report these ages as lower limiting estimates only; the removal of rock and the associated cosmogenic nuclides by weathering and ero-sion means that these surfaces could be far older than their model exposure ages suggest. Model ages for samples collected from bedrock surfaces between the prominent terraces; (n = 22) range from 45.8 ± 4.9 ka to 15.3 ± 1.6 ka, and in general increase in age with height above the channel fl oor.

Return to Rt. 372 and head east, recrossing the Susquehanna River and continuing for ~1 mi. Turn left onto River Road, drop into Kellys Run, and ascend the interfl uve. At the top of the hill, turn left onto Pinnacle Road, and continue to its end. Park at the locked gate and walk to the overlook.

STOP 2.4. Pinnacle Overlook

The purpose of this stop is to observe the deepest part of the Piedmont gorge, discuss its origin, introduce the terrace stratig-raphy for the Piedmont reach, and summarize the tectonic and landscape evolution model for the Atlantic margin. This stop overlooks the deepest portion of the Piedmont gorge (~200 m) at Holtwood. The river here is actually part of the Lake Aldred pool behind the Holtwood dam. Deep gorges like this one and steep, convex river long profi les are typical of all of the major streams that traverse the Piedmont and Fall Zone of the middle Atlantic States (Fig. 7). These features probably represent river incision in response to some combination of eustatic fall and/or epeirogenic uplift. The elevation and incision of the Piedmont here has been used to argue for broad, recent arching of the Piedmont (Camp-bell, 1929; Fig. 10), an explanation foreshadowing a more recent fl exural isostatic model of margin deformation (Pazzaglia and Gardner, 1994).

Flat uplands fl anking the river through the Piedmont reach are locally mantled by a rounded gravel lag representing the rem-nants of strath terraces (Fig. 11). There are three upland grav-els called Tg1, Tg2, and Tg3. They have similar texture with the possible exception of Tg1 which tends to be coarser grained and perhaps a bit more angular. The upland gravels are dominated

Figure 8. (A) Air photo, (B) cross section, and (C) long profi le of the is-lands at Holtwood. The white line in the air photo represents the location of the cross section, east on left, west on right. The white box in the pho-to represents a swath profi le of 10 m resolution digital elevation model (DEM) topography that is used to reconstruct the mean elevations of the islands through the Holtwood gorge. The numbered strath levels in the bottom cross section roughly correspond to the 4 straths of Reusser et al. (2006) shown below in Figure 9; however, Reusser et al. (2006) do not argue for upstream-dipping straths as depicted in this diagram.

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by resistant rock types such as vein quartz, quartzite, and quartz sandstone; however, the amount of quartz sandstone increases with decreasing stratigraphic age. Tg1, Tg2, and Tg3 are petro-graphically distinct enough to use clast counts as a means of cor-relation along the river. They are also much less heterolithic than QTg which is only locally preserved through the Piedmont reach, typically at the confl uence of major tributaries.

The geomorphology and Coastal Plain stratigraphy of the lower Susquehanna River and upper Chesapeake Bay region pro-vide the fi eld constraints (Fig. 12) for a geodynamic model of late Cenozoic deformation of the middle Atlantic passive margin. The details of these models are presented in Pazzaglia and Gardner (1994, 2000) and the interested reader is directed to these papers for full treatment of the methodology and results. A synthesis of all of the observations of the fi eld trip over the past several days can be summarized as follows.

• The early Tertiary Appalachian landscape was more deeply weathered and of lower local relief than the present topogra-phy. Climate change, epeirogenic uplift, or rapid increase in the size of the Atlantic slope drainage basin, or some combi-nation of all three factors, initiated the stripping of a mature regolith in the middle Miocene and its delivery to the Fall Zone (Fig. 13). Deposits accumulated during eustatic highs as the Bryn Mawr Formation with the upstream equivalents being preserved as the upland gravel terraces.

• Increased sediment fl ux to the Baltimore Canyon Trough (BCT), coupled with erosional unloading caused fl exure of the margin with the Fall Zone located at the fl exural hinge. A 1-dimensional line load model suggests an effec-tive elastic thickness of 30–40 km and long-term average erosion rates of ~10 m.y. (Fig. 14; Pazzaglia and Gardner, 1994). A more sophisticated 2- dimensional distributed load model using the same elastic thickness and erosion rates confi rms the Fall Zone as the fl exural hinge and offers an explanation for a mechanism locating the drain-age divide at the crest of the fl exural fore-bulge (Fig. 15; Pazzaglia and Gardner, 2000).

• Continued fl exural warping in the Piedmont has arched the Miocene terraces and contributes, along with long-term eustatic drawdown to the convexity and incision of the Susquehanna River channel.

• The incised Appalachian landscape now delivers a more immature, heterolithic load to the Coastal Plain and BCT. This immature petrography is a refl ection of the incision along with the effects of repeated Quaternary glaciations in the basin.

Return to River Road, turn left. Proceed for ~3 mi to Martic Forge. Stay on River Road at the intersection, noting that it takes a jog to the left. Proceed for another 2 mi to the mouth of the

A B

C

Figure 9. (A) Ages, and (B) incision rates calcu-lated from straths in the Holtwood Gorge. White represents older than level 3, light gray is from level 3 to level 2, dark gray is from level 2 to level 1. (C) Time-transgressive upstream young-ing of strath level 2 (from Reusser et al., 2006).


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Figure 10. Arching of the Piedmont called the Westminster anticline proposed by Campbell (1929). The arching was based on correlation of the Piedmont uplands which locally were mantled with river gravel. (A–C) Long profi les of rivers (gray shaded), terraces (dashed lines), and the Pied-mont upland (solid black line) for the Schuylkill, Susquehanna, and Potomac rivers respectively. (D) Sketch map showing contours drawn on the Westminster anticline with elevations in one-hun-dred foot intervals.


Conestoga Creek. Turn right onto Conestoga Blvd and proceed for about a mile to Rockhill. Turn left and cross Conestoga Creek, proceed for ~0.5 mi to the bridge across Little Conestoga Creek. Turn right and park at the barn.

STOP 2.5. Modern Processes and Responses to Land Use in the Conestoga Creek Watershed

Beginning with the fi rst arrival of European settlers in the late 1600s to early 1700s, the Piedmont landscape was dra-matically and substantially altered, not only by forest clearing and agriculture, but also by extensive, widespread damming of streams to power mills. By 1840, Lancaster and Chester Coun-ties had nearly 800 dams with an average dam height of ~2.4 m. Given the generally low gradients of streams in the region, the impounded water behind these dams extended upstream ~1.5–2.5 km, literally transforming pre-settlement wetlands, bogs, streams, and fl oodplains into a series of slack water ponds linked by mill races. Sites with extensive geochronological work that includes the use of 137Cs and 210Pb isotopes indicate that ponds were fi lled with sediment by ca. 1870, about the time that water-powered milling was supplanted by steam power. Superimposed on ca. 1870 A.D. aggradational surfaces is a modern suburban/urban overprint that includes rail lines, sewer lines, parking lots, recreational areas, etc. As development continued into the

twentieth century, increased storm water runoff and hurricanes (e.g., Tropical Storm Agnes in 1972) coincided with the breach-ing of many of the old mill dams. While widespread stream bank erosion in the region is blamed on excess storm water runoff, the sites we will visit indicate that the accumulation of thick stacks of post-settlement alluvium behind dams, followed by base-level fall associated with dam breaching, is the main cause of deep incision and subsequent bank erosion.

The historic record of channel form and process over the past 300 years can be used to showcase examples of successful stream restoration techniques. At this Little Conestoga Creek site, up to 5 m of historic sediment accumulated along a 2-km-long pond. LIDAR data is used to trace the anthropogenic aggradational (mill pond) surface upstream along the W Branch of Little Con-estoga Creek and into tributaries that drain into the W Branch. Deep incision and extensive lateral bank erosion has exposed the buried pre-settlement landscape, and we will discuss the form of the original streams. This stop will enable us also to discuss the notion of hydraulic geometry of stream channel form, as these ideas were developed by geomorphologists in the mid-twentieth century at sites we now recognize to have been mill ponds like those examined on this trip.

Retrace route back out to River Road at Conestoga Creek Park. Turn right and follow River Road for ~8 mi to Columbia. Turn

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Figure 11. Representative cross section of the lower Susquehanna River valley showing the relationship between pre-Pleistocene upland gravels and Pleistocene terraces (from Pazzaglia and Gardner, 1993).


right on Chestnut Street (Rt. 462), then left onto Rt. 441. Ascend Chickies Rock, and descend into the town of Marietta. Proceed past Marietta and, on the west side of town at a crossroads called Rowenna, park by the large white storage tanks at the intersec-tion of River Road and Vinegar Ferry Road.

STOP 2.6. Pleistocene Terraces at Marietta

The purpose of this stop is to observe a fl ight of Pleistocene terraces and discuss their genesis. Pleistocene terraces are well preserved along this reach of the Susquehanna River. Pleisto-cene terrace stratigraphy along the Susquehanna River was fi rst investigated by Peltier (1949) who demonstrated that there are several alluvial fi lls underlying mapable treads extending all the way from the head of Chesapeake Bay to heads of outwash at the glacier margin. The precise stratigraphic and genetic rela-tionship between these terraces was more recently investigated by Engel et al. (1996). There are at least six Pleistocene terraces and one possibly pre-Pleistocene terrace remnant preserved here at Marietta (Fig. 16). Water well data and former gravel pits confi rm that treads Qt1 through Qt6 are underlain by sev-eral meters of stratifi ed sand and gravel. QTg is not a stratifi ed deposit, but rather a scattered lag of rounded clasts of Appala-chian provenance, with an occasional diabase boulder. QTg can be traced downstream all the way to the Coastal Plain where it projects both in elevation and in composition to the upper part of the Pliocene Pensauken Formation. If this correlation is cor-rect and QTg is late Pliocene (ca. 2.5 Ma), the incision rate for the river at this point is 20 m/m.y. Terraces Qt1 through Qt6 are

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Figure 13. (A) Cross section of the Baltimore Canyon Trough (BCT) and (B) the fl ux of eroded rock to the BCT over the past 180 m.y. (be-low). Note the pulses of sediment, followed by exponential decay as well as the large pulse of sediment in the past 20 m.y. (from Pazzaglia and Brandon, 1996). The geodynamic models presented in Figures 14 and 15 model the post–Middle Miocene sediment loading.


distinguished compositionally by containing granite and gneiss, rock types not found in the upper Susquehanna basin that could only have been introduced by glaciation. For this reason, Qt1 through Qt6 are generally held to be genetically related to glacial outwash. Soil chronosequences establish a correlation between the terraces here and those at the heads of outwash 150 km upstream. Qt4 through Qt6 have soils consistent with late Pleistocene (Wisconsinan) age. Qt3 has a signifi cant loess cap and soil development consistent with a pre-Wisconsinan, presumably Illinoian age. Qt2 and Qt1 are thought to be pre-Illi-noian age. A geophysical study is currently under way to more carefully investigate the nature of the straths underlying the Pleistocene terraces at this site. There are two competing ideas regarding the preservation of Pleistocene terraces along the river that the strath geometry may help resolve. One possibility is that the Susquehanna River has been incising throughout the Pleistocene, producing the accommodation space to preserve the terraces. Each pulse of glacial outwash provides the tools to carve the strath and then aggrade the terrace alluvium. The alternative explanation is that the Susquehanna River incised to its current elevation in the Pliocene or early Pleistocene and the terrace represent the uneroded, “wings” of thick alluvial fi lls related to glacial outwash. The latter idea hinges on the fact that more recent glaciations were either less severe or sourced less sediment than earlier glaciations.

Continue west on River Road for ~4 mi and turn left into the Penn-sylvania Fishing Commission river access point at Falmouth.

Optional STOP 2.7

Potholes and fl uvially sculpted diabase in the Susquhanna River channel. The purpose of this stop is to take advantage of low-water conditions to directly observe the bedrock channel erosion processes of potholing, abrasion, and plucking where the Susquehanna River channel narrows and fl ows orthogonal to the strike of a resistant Jurassic diabase sill.

Return to River Road and turn left, continuing west toward Har-risburg. Pass Three Mile Island Nuclear power plant, site of an accident and partial meltdown of a reactor core on 28 March 28 1979. Enter Middletown. River Road turns into South Union Street. Stay on South Union Street to center of town. Turn left onto West Main Street and proceed for about a half mile to the Rt. 283 Airport Connector road. Take the Airport Connector Road to Rt. 283, proceed west to the interchange for the Pennsyl-vania turnpike, Rt. 76 West. Enter the Turnpike, cross the Susque-hanna River, and proceed west toward Carlisle. Take Exit 226 to Rt. 11 north, to I-81 south. Proceed south on I-81 for 15 mi, exit-ing at exit 37. Proceed north and west toward Newville on Cen-terville Road. In less than a mile, turn left onto Oak Flat Road,

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Figure 14. Results of a 1-dimensional line load model (from Pazzaglia and Gardner, 1994).


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Figure 15. Results of a 2-dimensional fl exural model of post–Middle Miocene fl exural deformation of the Atlantic mar-gin (Pazzaglia and Gardner, 2000).

Figure 16. Pleistocene terraces at Marietta (from Engel et al., 1996). Note Qt3 and Qt2 labels reversed in legend. w—water well.


and proceed for less than a mile to Big Spring Road. Turn left and proceed to the Big Spring parking area.

Optional STOP 2.8. Big Spring and the Hydrology of the Great Valley

The purpose of this stop is to discuss the complex hydrol-ogy and landscape of the carbonate rocks in the Cumberland Valley (for details see Potter, 2001, Day 1, Stop 8). Big Spring (Fig. 17) has an average discharge of 16.8 Mgal/day, which is ~6 times what would be expected from infi ltration of rainfall on the surface drainage area for the spring. About 16% of the spring’s fl ow is from the surface drainage area. Becher and Root (1981) and Chichester (1991, 1996) summarize the groundwater hydrol-ogy of the Cumberland Valley and the former estimate the “Big Spring diverts 5 to 10 percent of the Yellow Breeches (Creek, to the south) to the Conodoguinet Creek.” They note that “headwa-ter tributaries of Yellow Breeches Creek, directly south of Big Spring lose water as they fl ow across the colluvium (at the base of South Mountain. Yellow Breeches Creek is usually dry for 1½ miles downstream from Brookside during summer and fall.”

The large discharge per drainage area at Big Spring appar-ently occurs because immediately south of Big Spring there is no groundwater drainage divide between Yellow Breeches and Conodoguinet Creeks (Chichester, 1996). The groundwater sur-face just slopes north beneath the surface divide. However, far-ther east there is a groundwater divide immediately north of Yel-low Breeches Creek, and there, springs fed by water from South Mountain emerge near the Creek. In a recent dye-tracing experi-ment (Lindsey et al., 2006), dye placed in a sinkhole near Ship-pensburg, 5 mi to the west of Big Spring, emerged at Big Spring some days later, implying groundwater fl ow parallel to strike as opposed to fl ow across strike, as inferred above. It is clear that the karst hydrology here is complex. How has it evolved as erosion has proceeded downward here over millions of years?

Retrace route to I-81 and enter the interstate heading north toward Carlisle. Take Exit 44 for the accommodations at the Super 8–Carlisle.

DAY 3. THE GREAT VALLEY TO THE GLACIAL MARGIN

The fi nal day of our fi eld trip will begin by looking at depos-its in the Great Valley, continue with a discussion of long-term landscape development at the Harrisburg water gaps, and then conclude with the landforms and deposits of the glacial margin along the North Branch of the Susquehanna River.

Exit Hotel and enter I-81 South. Continue to Exit 29 (King St). Turn left (south) to Cleversburg. Turn right (west) and go to Mainsville. Turn left (south) onto Linsay Lot Road and go to sign on right to Valley Quarries. Enter and turn immediately left on paved road to gravel driveway into pit.

STOP 3.1. Mainsville Quarries and Geology of the Great Valley

The purpose of this stop is to examine the intensely weathered gravel at the base of South Mountain, the “trapping” of such gravel by carbonate dissolution, and the infl uence of that dissolution on drainage basin evolution. The Valley Quarries Mainsville pit is in a complex of alluvial-colluvial sand and gravel deposits derived from South Mountain. These deposits occur on top of carbonate rocks as fanlike aprons 1–5 km wide (Clark, 1991) that gently slope away from the mountain front. Similar deposits lie along the west edge of the Blue Ridge from here to central Virginia. Topography on the deposits is complex because karst processes have modifi ed the aprons. This pit and two others nearby are one of the few places where these gravels are exposed. Much of the gravelly material is a diamicton (mix of all sizes) developed both by emplacement by debris fl ows and by weathering of both debris fl ow and fl uvial material after deposition. The details of the geomorphology here and the relationship of these deposits to the carbonates are sum-marized by Sevon (2001, Day 2, Stops 2 and 3).

The overall stratigraphy of the aprons (Fig. 18) is almost entirely interpreted from water well records. The thickness of the material on top of the carbonates is commonly 30 m (100 ft)

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Figure 17. Big Spring surface drainage basin and its relationship to Yel-low Breeches Creek to the south. Also shown are adjacent surface drain-age basins of Mount Rock Spring and Alexander’s Spring to the east.


and in places is as much as 120 m (400 ft) (Sevon, 1999). Thick-ness generally decreases away from the mountain front. In a few places, thick deposits of clay residuum from weathering of the carbonates are noted beneath the gravels. Beneath the gravel at Pond Bank, ~8 mi SW of here, Pierce (1965) described 43 m (140 ft) of clayey residuum and a lignite deposit that contained Late Cretaceous spores. Using an insoluble residue content of 10% for the carbonates, he inferred that 430 m (1400 ft) of car-bonates have weathered beneath the gravel to produce the resid-uum. The processes that preserve the gravels and residuum are straightforward, but complex in detail. Acidic runoff from South Mountain percolates down through the gravel to dissolve the carbonate rocks. Voids of 2–3 m (5–10 ft) in the carbonates are common in well records. As the carbonates dissolve, the land-scape above lowers, residuum is added to the bottom of the pile of surfi cial deposits, and karst depressions form on the gravel apron surface. The gravel mantle protects the residuum from sub-aerial removal. Some karst depressions remain for long periods of time, for cores from ponds in two depressions have basal dates of 14 ka (Watts, 1979) and 16 ka (Delano, et al., 2002). The age of the gravels is uncertain, but weathering of some of the grav-els is intense enough that some cobbles disintegrate to the touch, suggesting a mid to late Tertiary age (Sevon, 2001).

Carbonate dissolution controls a lot of the local landscape evolution in the Cumberland Valley portion of the Great Val-ley. For example, the level of karst development controls local accommodation space for the fan material and determines if Yel-low Breeches Creek will be the master drainage of the Great Val-ley. The carbonate under-drain appears to respond to base-level fall faster and more effi ciently than the part of the valley under-lain by shale (Conodoguinet Creek). So when the Susquehanna River incises, Yellow Breeches Creek effi ciently fl ushes detritus through it, the fans are short and steep, and Yellow Breeches has a competitive advantage for drainage in the Great Valley over Conodoguinet. But when the Susquehanna River has reached its base level of erosion, so does Yellow Breeches. The carbonate

under-drain backs up with detritus, the fans are broad and gentle, and the Conodoguinet has the competitive advantage. This is the case in the Cumberland Valley today. These roles are reversed in Virginia, where shale drainages like the Shenandoah headwaters are losing to the carbonate drainages such as the Maury, a James River headwater. That is because the base-level fall there is hap-pening now, and the base-level fall on the Susquehanna happened in the past.

Retrace route to I-81 and proceed north for 35 mi, cross the Susquehanna River, and take the fi rst exit (Exit 67) onto Rt. 322–22 north. Proceed for 1 mi to Fort Hunter Park.

Optional STOP 3.2. Susquehanna Water Gaps at Fort Hunter County Park

The purpose of this stop is to view the classic Susquehanna River water gaps north of Harrisburg (Fig. 19) and to review the suggested origins for such water gaps. The park buildings occupy a high point on the fl oodplain that just projected above the largest historic fl ood in June 1972, Tropical Storm Agnes (Page and Shaw, 1973). The high area is a remnant of a late Wisconsinan recessional outwash terrace level ~8 m (25 ft) above present low water level, Peltier’s (1949) Valley Heads terrace. Much of the city of Harrisburg lies on the 15 m (50 ft) high outwash terrace developed during the late Wisconsinan maximum. A post-glacial–aged abandoned channel separates this high point from the eastern bedrock wall of the water gap. A low berm or strath, 2–3 m high, runs along the edge of the present Susquehanna channel. The channel here is on bedrock, as it almost entirely is from the late Wisconsinan terminus to Chesapeake Bay.

The geomorphic puzzle here is how the river became entrenched across the ridges of resistant rock and why the water gap developed where it did. The various theories on the origin of this water gap can be placed in several groups (modifi ed from Sevon, 1989a, 1989b): (1) superposition—from an estuary on the Schooley peneplain (Davis, 1889) or from overall Cretaceous marine transgression (Johnson, 1931); (2) headward erosion along structural weaknesses (Ashley, 1935; Meyerhoff and Olm-stead, 1936; Thiesen, 1983; Hoskins, 1987); (3) resistant sand-stone strata were thinner here (Thompson, 1949; Epstein, 1966); or (4) position determined by the front of a large overthrust sheet in the Anthracite region to the northeast (Sevon, 1986). Proof or disproof of the various hypotheses is diffi cult because on the order of 9 km of rock has been removed since the Alleghenian deformation that produced the folds here, and structures at the present depth of erosion do not necessarily refl ect those at the level where erosion began (Sevon, 1989a, 1989b).

Return to Rt. 322-22 heading north and turn left, continuing for a few miles toward Dauphin. Exit at Dauphin onto Rt. 225. Follow Rt. 225 as it switchbacks up over Peters Mountain. Park at the top of the mountain at the Appalachian Trail parking lot.

Figure 18. Schematic cross section of the fl ank of South Mountain, irregular fan-like surface mantled by sand and gravel, and underlying limestone.


STOP 3.3. Accordant Ridges and Long-Term Landscape Evolution in the Appalachians

This stop provides us with an opportunity to discuss land-form evolution and the development of the topography of the Ridge and Valley Province in the central Appalachians. This is the landscape that inspired Davis to develop his landform evolu-tion model, “The Geographical Cycle” (Davis, 1899). The Ridge and Valley consists of apparently even-crested ridge tops and intervening valley surfaces that trend northeast-southwest par-allel to the strike of folded sedimentary rocks. These relatively accordant ridges and valley surfaces led Davis and other early workers (Campbell, 1903; Ward, 1930) to conclude that each sur-face represented an episode of uplift followed by either nearly complete planation (peneplanation), or planation interrupted by an episode of uplift (partial planation).

The “accordant” summits of Third Mountain and the ridges visible to the north and south were interpreted by Davis (1889; Davis and Wood, 1890) as the remnant of a former peneplain produced by the reduction of the Late Paleozoic Appalachian mountain range to near base level. This surface was named the Schooley peneplain. The lowland surface visible to the north and south has been named the Harrisburg peneplain (Campbell, 1903) and the carbonate lowland surface south of the city of Har-risburg, but not visible from this stop, has been referred to as the Somerville peneplain (Campbell, 1903).

Even though the planation papers were all based upon a model of erosion that cut across lithotypes and structure, one of the most interesting aspects of these papers is that the authors all noted that the erosion surfaces were more or less coincident with specifi c lithotypes (Germanoski, 1999). The clear relationship between lithology and landform served as the basis for Hack’s (1960, 1973, 1975, 1980, 1982) ideas on landform development in the Appalachians. This stop highlights those observations and follows that line of reasoning.

Although there are differences in the details of the geology between the area viewed from this fi eld stop and the Lehigh Val-ley in eastern Pennsylvania ~150 km to the east, the Lehigh Val-ley serves as an analog to this landscape. The bedrock there con-sists of the same or similar folded sedimentary rocks in the Ridge and Valley, and Reading Prong fault-bounded basement blocks composed of gneiss and other metamorphic and igneous rocks which are analogous to South Mountain south of Harrisburg.

The Lehigh Valley landscape assemblage consists of fi ve major terrains.

Terrain 1. Blue Mountain, visible as the third ridge to the south from this fi eld stop, forms the highest topographic feature in the region. Blue Mountain is underlain by the Silurian age Sha-wangunk Formation, a highly resistant quartzite sandstone and conglomerate. The average elevation of the summit-line measured along a strike-parallel topographic profi le is 424 m above mean sea level (amsl), and outcrop averaged elevation of 385 m (based on digital elevation model data [DEM]) with a maximum of 506 m amsl and a minimum of 91 m amsl at the Delaware Water Gap.

There is as much as 118 m of relief on the mountain crest exclud-ing the deep gaps formed by the Lehigh and Delaware Rivers and the ancient channel that formed the Wind Gap (Fig. 20).

Terrain 2. North of Blue Mountain, the landscape consists of typical Appalachian Ridge and Valley terrain with long, paral-lel, narrow-crested ridges alternating with narrow, strike-parallel valleys carved by streams exploiting the outcrop belts of weaker strata. The ridges north of Blue Mountain are underlain by sand-stones that are less resistant to erosion than the Shawangunk quartz-ite, and these ridges vary in elevation from ridge to ridge. Typical elevations range from 213 m to as much as 396 m amsl (these are actually discordant summits). The view to the north here shows similar sandstone ridges that vary in elevation from ridge to ridge and along their crestlines, but appear accordant to the eye.

Terrain 3. The topography drops precipitously down the south fl ank of Blue Mountain to a laterally continuous upland surface underlain by the Ordovician age Martinsburg shale, slate, and greywacke (Fig. 20). The average elevation of this surface

Susquehanna River

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Second MtPeters Mt

Figure 19. Oblique air photo looking northwest into the fi rst three of the fi ve water gaps of the Susquehanna River at Harrisburg.

Figure 20. Three major topographic surfaces in the Lehigh Valley, Blue Mountain, the Martinsburg Slate surface, and the surface underlain by carbonates. Each surface corresponds with a distinct lithology.


measured using a DEM is 198 m amsl. This surface is ~10–11 km wide from northwest to southeast and forms a portion of the Great Valley Physiographic Section. This surface extends along strike from the Lehigh Valley southeast into Maryland and northeast into New Jersey.

Terrain 4. The Martinsburg surface is separated topographi-cally from the next surface to the southeast by a prominent escarpment (Fig. 20). The low carbonate surface has an average elevation of 116 m amsl. This surface is underlain by Cambrian-Ordovician carbonates that are highly susceptible to dissolution and have the lowest resistance to erosion.

Terrain 5. The Great Valley Section is bounded on the south by the fault-bounded basement blocks of the Reading Prong Sec-tion. The Reading Prong terrain consists of round-crested hilltops separated by intervening valleys underlain by Cambrian-Ordovi-cian carbonates. Reading Prong hills occur as individual isolated hills when surrounded entirely by carbonates and in other cases as more continuous rolling uplands when the crystalline base-ment blocks have larger, more contiguous outcrop area (Fig. 21). Elevations of the Reading Prong terrain are highly variable and range from 332 m amsl to 110 m amsl. Of the major terrains or surfaces in the Lehigh Valley region, the Reading Prong terrain has the most inconsistent topography.

DEM data from the Lehigh Valley indicate that the Lehigh Valley landform assemblage refl ects differential erosion of rocks of varying resistance to erosion. This conclusion is advanced on the basis of the strong association between geology and topog-raphy and is similar to the conclusions of other workers study-ing Appalachian landforms in the recent past (Hack, 1960, 1975, 1980; Flint, 1963; Braun, 1989a).

The Fieldstop LandscapePeters Mountain (our fi eld stop) is the north limb of a plung-

ing syncline and is underlain by the Devonian-age Duncannon Member of the Catskill Formation (a quartz sandstone). The fi rst

ridge to the south (Stony Mountain) is underlain by the Penn-sylvanian-age Pottsville sandstone, and the second ridge to the south (Second Mountain) is the south limb of the syncline we are standing on and is also underlain by the Duncannon Member. The third ridge to the south is Blue Mountain, the same landform as described in the Lehigh Valley along strike to the northeast, underlain by the resistant Tuscarora quartzite (the stratigraphic equivalent of the Shawangunk in the Lehigh Valley).

DEM data for landforms grouped by similar lithologies also shows systemic relationships between rock type and average topographic elevation and leads to similar conclusions as those drawn from the Lehigh Valley landscape. The sedimentary units are more numerous and lithologically diverse here as compared to the Lehigh Valley; therefore, lithotypes were placed into seven groups based on lithostratigraphic characteristics. Figure 22 shows the equivalent of frequency histograms of the elevation distributions in this area. The valley to the north is underlain by the Sherman Creek mudstones with a DEM average elevation of 194 m. Peters and Second Mountains, underlain by the resistant Pocono sandstone, have a crestline elevation of approximately 395 m, Third Mountain, underlain by the Pocono sandstone, has a crestline elevation of 495 m. These ridges are also discordant in elevation, as are the intervening valleys (Fig. 22). Notice that sandstone ridges vary in elevation from mountain to mountain and that there is a close correlation between lithotype and topog-raphy. This further supports the interpretation that landforms in the central Appalachians result from differential erosion of rocks having varying resistance to erosion.

Return to Rt. 225 and descend Peters Mountain toward Dauphin. At the second switchback, bear right onto Mountain Road and take it down to the intersection with Rt. 22-322. Enter Rt. 22-322 west, cross the Clark’s Ferry bridge, and within a mile, exit right onto U.S. 11 and 15 North. Stay on U.S. 11 and 15 North, fol-lowing the main stem of the Susquehanna River along its west bank. At Selinsgrove, we will cross the pre-Illinoian >788-ka-aged glacial limit. North of Shamokin Dam, turn right where U.S. 15 leaves the river valley and U.S. 11 continues along the river’s west bank with Sunbury and its high fl oodwall on the east bank. Cross the West Branch Susquehanna River on U.S. 11, and con-tinue east along the North Branch Susquehanna River. After pass-ing through Danville, U.S. 11 leaves the river valley and continues eastward along the fl oor of a strike valley underlain by shale and limestone. This valley should have been deeply fl ooded by Shaw’s (1989) proposed sub-glacially derived catastrophic fl oods.

Optional STOP 3.4A. The Grovania Divide, Intensely Weathered Pre-Illinoian–Aged Glaciofl uvial Deposits Where a 14 ka-Aged 50 m Deep Catastrophic Flood Supposedly Flowed

Time permitting, we will stop here to briefl y view the very “unscabland-like” landscape mantled by pre-Illinoian gla-cial deposits (Fig. 23, Site GD). The site is the local drainage divide on the fl oor of a linear strike valley underlain by shale and

Figure 21. Discrete ridges underlain by more resistant rocks such as the Martinsburg Formation or Middle Proterozoic crystalline rocks. Lowland surface in middle ground is underlain by carbonates.


300

Figure 22. Average and crestline elevations of the major topographic/lithostratigraphic landforms at the Peters Mountain stop.

Figure 23. Map of the Bloomsburg area showing the present Susquehanna River (SR), its blocked and abandoned course (SR ar-rows), buried course of Fishing Creek (FC arrow) and the late Illinoian or older glacial limit (LI). Other features are: B—Blooms-burg (Stop 3.4B), CC—Catawissa Creek, FC—Fishing Creek, GD—Grovania Divide (Stop 3.4A), ID—Illinoian glacial deposits, OW—outwash terraces and alluvium, PID—pre-Illinoian glacial deposits, RG—Rupert water gap, RHU—rolling hill uplands, SV—strike valley, STR—strike ridge.


limestone. This valley is the “catastrophic fl ood bypass route” for the “undersized” North Branch Susquehanna River water-gap at Bloomsburg (Stop 3.4B). The gently rolling landscape on the ~2 km wide valley shows no sign of fl ood scour features. Much of the fl oor of the valley is underlain by pre-Illinoian–aged glacio-fl uvial and glacial till materials that retain a 1–10-m-deep weath-ering profi le. Along the fl anks of the valley, the older deposits are buried by Wisconsinan-aged colluvial deposits derived from the adjacent sandstone ridges. These materials would be expected to be deeply scoured or entirely removed if a catastrophic fl ood had passed this way in latest Wisconsinan times. With luck, there may be a temporary excavation available to examine.

Continue east on U.S. 11 into Bloomsburg. Continue straight uphill where U.S. 11 turns left. Make a right then left onto 2nd Street, continue uphill, and park on left next to Bloomsburg Uni-versity’s Hartline Science Center.

STOP 3.4. Overview of the Susquehanna Valley and the Question of Catastrophic Sub-Glacial Floods

The purpose of this stop is to discuss the evidence for cata-strophic, channeled scabland scale fl oods coming down the North Branch Susquehanna valley. Shaw (1989) proposed that the Susquehanna valley be examined for evidence of catastrophic discharges on the order of 4.8 × 106 m3/s. Such discharges suppos-edly come from 14−16 ka-aged sub-glacial fl oods that formed the drumlins and helped carve the Finger Lakes in New York State.

Directly below our vantage point are the North Branch Susquehanna River fl oodplain and outwash terraces covering a shale and limestone strike valley (Fig. 23, OW and SW). The Trimmers Rock Formation sandstone ridge on the south side of the valley is being undercut by the river today. On the south skyline is Catawissa Mountain, underlain by Pocono Forma-tion. Sandstone (Fig. 23). The Trimmers Rock ridge to the east is cut by the 1.5 km wide abandoned North Branch Susquehanna valley, blocked by late Illinoian or older Bloomsburg ice mar-gin deposits (Fig. 23). Between there and here was the location of the pre-glacial low divide in the strike valley that separated the Susquehanna River from Fishing Creek. Pre-glacial Fishing Creek once passed through the ridge behind us in a water gap 2 km east of here that was also buried by Bloomsburg ice margin deposits (Fig. 23).

To the southwest, the river turns a 90° angle and enters the 0.5 km wide Rupert water gap (Fig. 23, RG). The narrowness of the gap refl ects its origin as the former gap of the 30 m wide Fishing Creek. It has been occupied by the river only since ca. 150 ka, when the late Illinoian glaciation (Braun et al., 1984; Braun, 1994a, 1989b) diverted the river down the strike valley below us and into the Rupert gap.

To the right of the Rupert water gap is the continuation of the strike valley. To the west is a low divide at Grovania that is only 50 m above the present fl oor of the Rupert gap. The strike valley is a “straight ahead” bypass for catastrophic fl oods ponding at

the constriction presented by Rupert gap (Fig. 23). The Rupert water gap is also exceptionally shallow, 140 m, because the river is cutting the Trimmers Rock ridge, a lower ridge compared to the higher strike ridges such as nearby Catawissa Mountain. Due to this combination of a low strike ridge and a uniquely youthful age, the Rupert water gap has less than one-half the cross-sec-tional area (0.74 × 105 m2) of any water gap upriver of it. This “undersized” Rupert water gap is the critical “choke-point” for hypothesized catastrophic fl oods. Braun (1990) estimated only 25% of the proposed catastrophic fl ood would fi t through the Rupert gap and the rest would fl ow down the adjacent strike val-ley at a depth of 50 m or more. That valley should show dramatic evidence of scabland-like erosion features but no such features are observed. Instead, deeply weathered pre-Illinoian glacial depos-its (Stop 3.4A, Fig. 23) mantle the valley fl oor, often capped by Wisconsinan colluvium or loess (Leverett, 1934; Peltier, 1949; Braun, 1990). There is no evidence for catastrophic fl ooding.

Leave Bloomsburg University on Rt. 487 N, take I-80 east and immediately cross the buried valley of Fishing Creek, Cross Susquehanna River, exit I-80 onto Rt. 339, and turn left through Miffl inville. Travel on Susquehanna River outwash and alluvial terraces to Nescopeck. Turn left then right onto Rt. 93, leave Nescopeck, and bear left on Wapwallopen Road. Ascend riser onto outwash terrace and stop at gravel pit on right.

STOP 3.5. The Late Wisconsinan Terminal Head-of-Outwash in the Susquehanna Valley

The purpose of this stop is to examine the landforms and deposits that characterize the terminus of the Laurentide ice in the Ridge and Valley of eastern Pennsylvania. The site is a head of outwash or frontal kame in the Susquehanna River valley. The sequence of deposits coarsens upward and becomes enriched in locally derived clasts as ice gradually approaches the site (Fig. 24). The ground surface exhibits knob and kettle topogra-phy with a thin loess mantle. The kame fan was gradually incised 35 m by increasing meltwater fl ow as ice retreated northward and added increasing ice-front drainage area. Meltwater drainage ceased at ca. 14 ka, when ice retreated north of the Susquehanna basin. Since then the river has incised 6 m more through the out-wash and is now incising into bedrock.

Continue east along Susquehanna River to Rt. 239 and Wapwal-lopen. Turn right onto Hobbie Road, ascend slope and follow signs to Council Cup Park, making several turns.

STOP 3.6. Council Cup Overview of the Three-Tier Landscape: Davis, Hack, and More Recent Explanations

The purpose of this stop is to view the three-tier landscape in the Ridge and Valley (Fig. 25) and review past and current explanations of how that landscape developed. There have really been only two explanations for the Appalachian landscape,


Figure 24. Stratigraphic section of the late Wisconsinan terminus in the Susquehanna valley. (from Braun and Inners, 1988).

Figure 25. Schematic cross section of the breached Berwick anticline showing the relation-ship of bedrock units to the three-tier landscape and postulated peneplains.


Davis (1889, 1899) and Hack (1960, 1965). Davis explained the three-tier landscape as being remnants of three peneplains (low relief fl uvial erosion surfaces) of differing age (Fig. 24). Davis thought the highest peneplain developed during the Cretaceous and named it the Schooley peneplain (Davis and Wood, 1890). That peneplain was uplifted in the early Tertiary and a partial peneplain developed during the middle Tertiary (named the Har-risburg peneplain by Campbell, 1903). A second uplift occurred in the late Tertiary and then a second partial peneplain developed locally on the weakest rock (named the Somerville by Davis and Wood, 1890). This explanation requires erosion to carve out only the present relief of ~500 m since the Cretaceous. Later work-ers mainly added to the number of peneplains (Bascom, 1921), extended the Coastal Plain cover (Johnson, 1931) or made the peneplains younger (Johnson, 1931; Ashley, 1935).

Hack explained the three-tier landscape a being a result of continuous erosion working on rocks of three differing resis-tances that produces the three different elevations. The hardest sandstone forms the highest ridges, interbedded sandstone and shale form an intermediate elevation rolling hill landscape, and the shale and limestone units form the gently rolling lowest ele-vations (Fig. 25).

The crucial test of Davis’ theory is to examine the amount of sediment eroded from the Appalachians and deposited in the Coastal Plain and offshore. Long-term rates of post-Triassic ero-sion have been reconstructed from the offshore sediment load in the Baltimore Canyon Trough (Poag and Sevon, 1989) (Fig. 13). In summary, exhumation was relatively fast in the Late Cretaceous (>30 m/m.y. assuming constant drainage basin size), very slow in the Tertiary (<30 m/m.y. assuming constant drainage basin size), and then very fast in the Miocene, continuing to the present. The slug of Miocene-Recent sediment in the BCT requires removal of at least 1–2 km of rock in the past 20 m.y (Braun, 1989b; Paz-zaglia and Brandon, 1996). This makes it unlikely that a pre-Mio-cene peneplain could be preserved in the present landscape.

It has been suggested that the Schooley peneplain is of Miocene or even Pliocene age (Ashley, 1933, 1935). But Davis implied (although never directly stated) that peneplanation takes tens of millions of years. Later workers, like Schumm and Lichty (1965) and Pitman and Golovchenko (1991), also argue that peneplanation takes tens of millions of years to complete. So there is just not enough time to form a peneplain in Miocene to Pliocene times, let alone enough time to incise the fi rst one and develop a second one.

Current erosion rates are still high compared to rates in the early to mid Tertiary and early to mid Cretaceous (Fig. 13). His-toric river suspended sediment yields suggest erosion rates between ~20 and 40 m/m.y (Sevon, 1989a; Milliman and Syvitsky, 1992; Conrad and Saunderson, 1999), but these values are somewhat high from human-induced soil erosion. Historic solute loads range between ~5 and 10 m/m.y. (Cleaves et al., 1970; Cleaves, 1993). Quaternary periglacial erosion of ridge tops may be as high as ~80–100 m/m.y. (Braun, 1989a). Carbonate dissolution rates typi-cally range between ~8 and 30 m/m.y. (White, 1984, 2000). Soil

production and mean residence times indicate rates ~10 m/m.y. (Pavich et al., 1989). River incision rates range from 5 to 40 m/m.y. (Pazzaglia and Gardner, 1994; Granger et al., 1997, 2001; Mills, 2000). The erosion rate from cosmogenic dated surfaces averaged across entire watersheds in the Smoky Mountains of the southern Appalachians is 27 m/m.y (Matmon et al., 2003). Ero-sion rates vary across the Appalachian landscape depending on local relief, rock type, and proximity to the Fall Zone, but overall the Appalachians are currently unroofi ng at rate of ~20–30 m/m.y. Such high current erosion rates indicate that, since the mid-Mio-cene erosion maximum, the landscape has not yet approached the low erosion rates expected on a low relief peneplain surface.

This continuous, rapid mid-Miocene to present erosion is what has formed the three-tier Ridge and Valley landscape (start-ing 1 km or so above the present ridge crests), and that continuity of erosion is what Hack emphasized in his later work (1975). It is the dynamic equilibrium part of Hack’s model that is most prob-lematic. Uncertainty in measuring basin-wide erosion rates and river-specifi c incision rates makes it diffi cult to know if the Appa-lachian landscape is maintaining relief in a dynamic equilibrium state (Hack, 1960), or if relief is locally increasing or decreasing. Little separation between the straths of successive glaciofl uvial terraces on the Susquehanna suggests that the river is incising slower than periglacial activity is eroding the hilltops, essentially reducing relief in the landscape. However, close to the Fall Zone, the river profi le is clearly convex, indicating base-level fall and active incision. Process rates are slow in this landscape, and there are signifi cant lag times (Pazzaglia et al., 1998), making it very diffi cult to determine whether the Appalachian landscape is in dynamic equilibrium or in some disequilibrium state.

Looking back further in time at the entire post-Triassic sedi-ment yield curve (Fig. 13) shows that there were two earlier pulses of clastic sediment delivered to the BCT, each followed by an exponential decrease to a very low sediment yield. This fi ts the Davisian model of landscape evolution and also fi ts with two major unconformities at the edge of the Coastal Plain, one underneath the late Cretaceous sediments and the other under the mid-Miocene sediments as was discussed on the fi rst two days of the trip.

Even before the late Jurassic sediment pulse shown on Fig-ure 13, there had been deep unroofi ng of the Appalachians in the Triassic as shown by the erosion surface under the rift basins and by the thermochronologic data. Much of that eroded material was shed in a vast alluvial plain to the west and north, the direction of drainage at the close of the Alleghenian orogeny in the Per-mian. Triassic continental rifting rejuvenated the Appalachians, opened the Atlantic Ocean and began the reversal of the drainage to the east. The exponential decline in sediment yield from the Late Jurassic to the close of the Early Cretaceous (160–100 Ma) (Fig. 13) is a result of the Triassic rift topography being reduced and beveled to form the Fall Zone low erosion surface (uncon-formity). Davis interpreted that surface to be a peneplain. A Late Cretaceous (ca. 90 Ma) transgression then brought fl uvial-deltaic and possibly shallow marine deposits across Fall Zone the and possibly farther west.


The Late Cretaceous sediment pulse (Fig. 13) indicates renewed uplift and erosion in the Appalachians for a yet unknown reason. That uplift was followed by another exponential decline in sediment yield that lasted for 65 million years from the lat-est Cretaceous to the early Miocene. This decline in sediment yield is related to a second reduction in relief in the Appalachians toward a low relief erosion surface, the Schooley surface. Davis interpreted that surface to be a peneplain, other possible inter-pretations is that it is a pediplain or an etchplain. From limited subsurface information, both the earlier Fall Zone and Schooley unconformities have relief similar to the present Fall Zone land-scape (Gardner et al., 1994), suggesting there were never truly low relief erosion surfaces in the Appalachians.

Recent work supports an early Miocene age for Schooley surface in the New Jersey Highlands west of the Coastal Plain (Stanford, 1997; Stanford et al. 2000, 2001). Along the Susque-hanna River, the Piedmont upland surface next to the Coastal plain is also considered to be of Miocene age (Pazzaglia and Gardner, 1994), as was discussed on day two of this trip. It is expected that the area next to the hinge line between erosion and deposition could retain remnants of an erosion surface dating from before the mid-Miocene sediment pulse. But again, to account for the volume of sediment produced, much greater erosion is necessary upstream. It requires that erosion has removed all vestiges of a Miocene-aged erosion surface farther inland.

New U-Th/He thermochronologic data and existing apatite fi ssion track (AFT) thermochronology reveals a denudation pat-tern consistent with a variable geothermal gradient and a system-atic, recent shift in the drainage divide linked to a similar shift in the depositional basins accumulating unroofed sediments on the Coastal Plain (Fig. 26). In the central Appalachians, U-Th/He and AFT closure ages are virtually identical for the Ridge and Valley province and suggest very modest amounts of unroofi ng in the past ~200 m.y. In contrast, the U-Th/He ages are systemati-cally younger than AFT ages to the east in the Piedmont and Blue Ridge provinces, suggesting more and continuous unroofi ng dur-ing Alleghenian orogenesis and Mesozoic rifting. The problem with these data, particularly in Pennsylvania, lies in the fact that the largest amount of unroofi ng is sensitive to the geothermal gra-dient. Rifting likely elevated the gradient in the Blue Ridge and Piedmont and if steep gradients approaching 50 °C/km are con-sidered, then the amount of section taken off the Blue Ridge and Piedmont is commensurate with that taken off the Ridge and Val-ley over the past ~150 m.y. Overall, the long-term rate of erosion over the past 150 m.y. has been ~20 m/m.y. This rate compares well to studies of sediment yield in the Juniata basin (27 m/m.y., Sevon, 1989a) and the new results of cosmogenic erosion rates throughout the Susquehanna basin of (J. Reutter, 2006, personal commun.). One way to reconcile the thermochronology, cosmo-genic, and sediment yield data, with the offshore sediment record (Fig. 13) is to argue that the Miocene sediment pulse represents an unsteadiness in the growth of Atlantic slope drainages. In essence, a westward jump of the drainage divide across the Ridge and Valley to its current location on the Allegheny Plateau in the

Miocene would deliver more sediment to the margin as relief in the Ridge and Valley was increased, but the overall depth of exhu-mation has not been great enough to see young, reset ages at the surface. Similarly, the sediment pulse may indicate extra-basinal sources, such as the unroofi ng of New England and recycling of its former Coastal Plain by the former Connecticut and Hudson Rivers (Pazzaglia, 1993). Or the sediment pulse may even rep-resent rapid erosion throughout the present Susquehanna Basin of a very deep weathering profi le developed during the previous 40 Ma of low sediment yield.

Return to Wapwhallopen, turn left onto Rt. 239 then bear left upslope on Rt.239. Cross kettled kame terrace and then “indis-tinct” terminal moraine. Turn right continuing on Rt. 239. Turn left onto Rt. 93 and ascend Nescopeck Mountain. At top, bear right onto Old Berwick Road and descend mountain. Near base of mountain, turn left into phone company parking lot.

Optional STOP 3.7 Late Wisconsinan over Illinoian Colluvium (Time Permitting)

The purpose of this stop is view two different ages of col-luvium. The site is on toe of the dip-slope of a Pocono sandstone ridge. Late Illinoian or older ice covered the site. The land sur-face is covered by a one-stone-thick cobble and boulder mantle that is sorted into crude stripes of stone-rich and stone-poor soil.

Coo

ling

Age

(M

a)

0

50

100

150

200

250

300

Roden and Miller, 1989AFT

RVU-Th/He

BRU-Th/He

PU-Th/He

Kohn et al., 1993ZFT

8

9

15

89

Figure 26. Comparison of thermochronometric cooling ages for parts of the Pennsylvania foreland and rift shoulder. RV—Ridge and Val-ley, BR—Blue Ridge, P—Piedmont, AFT—apatite fi ssion track, and ZFT—zircon fi ssion track. Numbers in boxes correspond to the num-ber of samples. Dashed lines are means, solid lines are modes, whis-kers are the 5th and 95th percentile, dots are outliers. The Roden and Miller (1989) study includes data from the Ridge and Valley and the Kohn et al. (1993) study includes data for the Piedmont.


The upper one-half meter or so of material has a brown color that is typical of late Wisconsinan-aged material. The thinness of the brown material suggests that this location was a transport zone supplying colluvium to a deposition zone farther downslope. The upper colluvium is underlain by a weathered reddish-brown colluvium derived from the glacial drift. The well-developed downslope clast fabric indicates the material is colluvium. Ice fl ow was parallel to the slope and would have left a fabric perpen-dicular to what is observed.

Turn left upon leaving. At Rt. 93, turn right then right onto I-80 east. Cross I-81 and late Wisconsinan terminus. Exit onto Rt. 309 north and ascend ridge, crossing terminus again. Turn left at

traffi c light onto East Butler Road and ascend Green Mountain. Near top, turn left onto Prospect Road, cross Anthracite coal strip mines, go through Upper Lehigh, cross other coal strips, and park on right shoulder beside pond in reclaimed strip mine. Walk east to lake in strip mine. Exposures are on the north side of lake.

STOP 3.8 Late Illinoian Terminus Deposits

The purpose of this stop is to examine older glacial depos-its capped by late Wisconsinan colluvium and discuss the amount of erosion since the glacier retreated. Table 1 presents a stratigraphic column with a description and interpretation of the deposits.

TABLE 1. STRATIGRAPHIC COLUMN OF GLACIAL AND COLLUVIAL DEPOSITS EXPOSEDALONG THE LATE ILLINOIAN TERMINUS IN THE ANTRACITE FIELDS OF EASTERN PENNSYLVANIA

M L DESCRIPTION OF MATERIAL INTERPRETATION

6–9 12 Reddish brown; diamict with clasts of rock, tree, and anthropogenic debris; lower contact is a truncated ground surface with the original colluvial mantle removed in places.

Strip mine waste pile

1–3 11 Conglomerate boulder line underlain by yellow brown diamict with abundant quartz pebbles; near base reddish brown diamict material incorporated from underlying material.

Late Wisconsinan colluvium

3 10 Reddish brown; bouldery, clayey sand to clayey silt matrix diamict; compact with tabular clasts showing slight N-NE dip; clasts are dominantly Mauch Chunk redbed material, angular to subrounded, striated, and many show some weathering, boulder concentration in lower 5 ft; lower contact is sharp.

Basal till, fi nal readvance of late-Illinoian ice to terminus

3.5 9 Reddish yellow to reddish brown; pebbly to cobbly silty to clayey sand diamict grading down ward into a pebbly to cobbly compact sand ; matrix in upper part displays a well developed near-horizontal fi ssility or play structure, becomes blocky below; no boulders; no striated clasts; Mauch Chunk clasts common but not dominant; lower contact transitional over 2–3 cm.

Sub-lacustrine debris fl ows from advancing late Illinoian ice

1.5 8 Light brown in upper 3 ft, light brownish gray below; gravelly sand; friable; sparse redbed clasts; forms top surface of a bench; forms light brown stripe across the outcrop face; lower contact is sharp.

Sub-lacustrine outwash, initial ice readvance

1 7 Reddish brown; clayey silt; massive to laminated; at lower contact the material is draped over underlying boulders.

Proglacial lacustrine sediment

1.8 6 Reddish yellow to reddish gray with black laminae; bouldery sand to silty sand; silty material more grayish; boulder concentration near base; forms a lighter brown stripe across the outcrop face; lower contact gradational over 2–4 cm.

Sub-lacustrine outwash

2.7 5 Reddish brown to reddish gray; interbedded bouldery sand, sand, and silty sand; silty material more grayish with more redbed grains; cross-bedding dips SW; gray sandstone clast surfaces strongly oxidized, quartz conglomerate and redbed clast surfaces less oxidized; a few striated clasts; bedding curves under some clasts; lower contact gradational over 5–10 cm.

Sub-lacustrine outwash

1.5 4 Reddish brown; pebbly to cobbly, clayey sand to clayey silt matrix diamict; matrix becomes sandier upward; redbed clasts dominant near base; a few striated clasts; thickens to 18 ft eastward 400 ft; lower contact transitional over 2–5 cm.

Till from fi rst late Illinoian advance to terminus

0.5 3 Reddish gray; clayey sand to clayey silt matrix diamict; forms a grayish band across the outcrop face; clasts are dominantly pebble sized; lower contact transitional over 5–10 cm.

Debris fl ows from advancing ice

2.4 2 Reddish gray to brown; pebble gravel with sand stringers; rounded to platy clasts; horizontal strata with Clast imbrication indicating fl ow from N to NE; redbed pebbles dominant at base, subequal to quartz and sandstone pebble upward; Black bands and patches of Fe and Mn cementation; lower contact transitional over 2 cm.

Colluvium deposited primarily by slope wash, lower part from removal of pre-Illinoian drift

0.6 1 Light gray to pinkish gray; pebbly to cobbly, clayey silt matrix diamict; moderate red to pale red purple, angular tabular clasts from underlying bedrock; light to medium gray quartz and sandstone clasts from upslope; lower contact sharp.

Mixed colluvium and residuum of pre-Illinoian age

Micaceous sandstone, claystone, and coal; dips steeply to SE, upper surface weathered with widened joints.

Llewellyn Formation bedrock

Note: Modifi ed from Braun (1999).


Continue ahead bearing right, turning right then left to take Rt. 940 toward Hazleton. Turn right on SR 3019 and cross the active Anthracite strip pits. At Rt. 93, turn right on 93. Turn left onto U.S. 209 to go through Jim Thorpe and Lehighton. Follow 209 to I-476, continue to center city Philadelphia.orReturn to I-81 and go east to I-476, continue to center city Phila-delphia.FIELD TRIP ENDS

ACKNOWLEDGMENTS

We would like to thank Joan Ramage for providing a careful review of the manuscript.

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