SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 129

D

da dt

Overturnedflap

Overturnedflap

True crater

True crater

Apparent crater

Faultedrim

Faultedrim

Breccia and impactmelt annulus

Breccia and impactmelt annulus

Central peakand rings

( ) Complex impact structureb

( ) Simple impact structurea

1 km

Sedimentary fill

Allochthonous fallback breccia

Fallout ejecta

Impact melt and melt fragments

Strongly shocked target rocks

Fractured and brecciatedtarget rocks

Fault

Figure 5.11 Simple and complex impact structures.

new impact sites every year. Researchers have also foundseveral impact structures in the seafloor.

The spatial distribution of terranean impact structuresreveals a concentration on the Precambrian shield areasof North America and Europe (Figure 5.12). This con-centration reflects the fact that the Precambrian shields inNorth America and Europe have been geologically stablefor a long time, and that the search for, and study of,impact craters has been conducted chiefly in those areas.It is not a reflection of the impaction process, whichoccurs at random over the globe.

LANDFORMS ASSOCIATEDWITH FOLDS

Flat beds

Stratified rocks may stay horizontal or they may befolded. Sedimentary rocks that remain more or less

horizontal once the sea has retreated or after they havebeen uplifted form characteristic landforms (Table 5.2).If the beds stay flat and are not dissected by river val-leys, they form large sedimentary plains (sediplains).Many of the flat riverine plains of the Channel Country,south-western Queensland, Australia, are of this type.If the beds stay flat but are dissected by river valleys, theyform plateaux, plains, and stepped topography (ColourPlate 1, inserted between pages 208 and 209). In sedi-mentary terrain, plateaux are extensive areas of low reliefthat sit above surrounding lower land, from which theyare isolated by scarps (see Figure 5.16, p. 135). A bedof hard rock called caprock normally crowns them. Amesa or table is a small plateau, but there is no finedividing line between a mesa and a plateau. A butteis a very small plateau, and a mesa becomes a buttewhen the maximum diameter of its flat top is less thanits height above the encircling plain. When eventuallythe caprock is eroded away, a butte may become an

130 STRUCTURE

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SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 131

Table 5.2 Landforms associated with sedimentary rocks

Formative conditions Landform Description

Horizontal beds

Not dissected by rivers Sediplain Large sedimentary plainDissected by rivers with

thin caprockPlateau Extensive flat area formed on caprock, surrounded by

lower land, and flanked by scarpsMesa or table Small, steep-sided, flat-topped plateauButte Very small, steep-sided, flat-topped plateauIsolated tower, rounded peak,

jagged hill, domed plateauResidual forms produced when caprock has been

erodedStepped scarp A scarp with many bluffs, debris slopes, and structural

benchesRibbed scarp A stepped scarp developed in thin-bedded strataDebris slope A slope cut in bedrock lying beneath the bluff and

covered with a sometimes patchy veneer of debrisfrom it

Dissected by rivers withthick caprock

Bluffs, often with peculiarweathering patterns

Straight bluffs breached only by major rivers.Weathering patterns include elephant skinweathering, crocodile skin weathering, frettedsurfaces, tafoni, large hollows at the bluff base

Folded beds

Primary folds at variousstages of erosion

Anticlinal hills or Jura-typerelief

Folded surfaces that directly mirror the underlyinggeological structures

Inverted relief Structural lows occupy high areas (e.g. a perchedsyncline) and structural highs low areas (e.g. ananticlinal valley)

Planated relief Highly eroded foldsAppalachian-type relief Planated relief that is uplifted and dissected, leaving

vestiges of the plains high in the reliefDifferential erosion of

folded sedimentarysequences

Ridge and valley topography Terrain with ridges and valleys generally following thestrike of the beds and so the pattern of folding(includes breached anticlines and domes)

Cuesta Ridge formed in gently dipping strata with anasymmetrical cross-section of escarpment anddip-slope

Homoclinal ridge or strikeridge

Ridge formed in moderately dipping strata with justabout asymmetrical cross-section

Hogback Ridge formed in steeply dipping strata with symmetricalcross-section

Escarpment (scarp face, scarpslope)

The side of a ridge that cuts across the strata. Picks outlithological variations in the strata

Dip-slope The side of a ridge that accords with the dip of the strataFlatiron (revet crag) A roughly triangular facet produced by regularly

spaced streams eating into a dip-slope or ridge(especially a cuesta or homoclinal ridge)

Source: Partly after discussion in Twidale and Campbell (1993, 187211)

132 STRUCTURE

isolated tower, a jagged peak, or a rounded hill, depend-ing on the caprock thickness. In stepped topography,scarps display a sequence of structural benches, producedby harder beds, and steep bluffs where softer beds havebeen eaten away (see Colour Plate 10, inserted betweenpages 208 and 209).

Folded beds

Anticlines are arches in strata, while synclines aretroughs (Figure 5.13). In recumbent anticlines, the bedsare folded over. Isoclinal folding occurs where a seriesof overfolds are arranged such that their limbs dip inthe same direction. Monoclines are the simple folds inwhich beds are flexed from one level to another. Anexample is the Isle of Wight monocline, England, whichruns from east to west across the island with Creta-ceous rocks sitting at a lower level to the north thanto the south. In nearly all cases, monoclines are veryasymmetrical anticlines with much elongated arch andtrough limbs. Anticlines, monoclines, and synclines formthrough shearing or tangential or lateral pressures appliedto sedimentary rocks. Domes, which may be regarded asdouble anticlines, and basins, which may be regardedas double synclines, are formed if additional forces come

( ) Anticlinea ( ) Synclineb ( ) Asymmetrical foldingc ( ) Monoclined

( ) Isoclinese ( ) Recumbent foldf ( ) Domeg ( ) Basinh

Figure 5.13 Structures formed in folded strata.

from other directions. Domes are also termed periclines.An example is the Chaldon pericline in Dorset, England,in which rings of progressively younger rocks WealdenBeds, Upper Greensand, and Chalk outcrop arounda core of Upper Jurassic Portland and Purbeck beds.Domed structures also form where the crust is thrustupwards, although these forms are usually simpler thanthose formed by more complex pressure distributions.Domes are found, too, where plugs of light material,such as salt, rise through the overlying strata as diapirs.

Folds may be symmetrical or asymmetrical, open ortight, simple or complex. Relief formed directly by folds israre, but some anticlinal hills do exist. The 11-km-longMount StewartHalcombe anticline near Wellington,New Zealand, is formed in Late Pleistocene sedimentsof the coastal plain. It has an even crest, the surfaces ofboth its flanks run parallel to the dip of the underlyingbeds (Box 5.1), and its arched surface replicates the fold(Ollier 1981, 59). Even anticlinal hills exposed by ero-sion are not that common, although many anticlinal hillsin the Jura Mountains remain barely breached by rivers.

The commonest landforms connected with foldingare breached anticlines and breached domes. This isbecause, once exposed, the crest of an anticline (or thetop of a dome) is subject to erosion. The strike ridges on

SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 133

Box 5.1

DIP, STRIKE, AND PLUNGE

In tilted beds, the bedding planes are said to dip. Thedip or true dip of a bed is given as the maximum anglebetween the bed and the horizontal (Figure 5.14a).The strike is the direction at right angles to the dipmeasured as an azimuth (compass direction) in thehorizontal plane.

Strike

Directionof dip

Angleof dip

( )a ( )b

Plunge

Figure 5.14 Terms relating to sedimentary structures. (a) Dip and strike. (b) Plunge.

An anticlinal axis that is tilted is said to pitch orplunge (Figure 5.14b). The angle of plunge is theangle between the anticlinal axis and a horizontal plane.Plunging anticlines can be thought of as elongateddomes. Synclinal axes may also plunge.

each side tend to be archetypal dip and scarp slopes, witha typical drainage pattern, and between the streams thatcross the strike the dipping strata have the characteris-tic forms of flatirons, which are triangular facets withtheir bases parallel to the strike and their apices pointingup the dip of the rock. The strike ridges are very longwhere the folds are horizontal, but they form concentricrings where the folds form a dome. The scarp and valesequence of the Kentish Weald, England, is a classic caseof a breached anticline (Figure 5.15). Strike ridges maysurround structural basins, with the flatirons pointing inthe opposite direction.

Where strata of differing resistance are inclined overa broad area, several landforms develop according tothe dip of the beds (Figure 5.16). Cuestas form in

beds dipping gently, perhaps up to 5 degrees. Theyare asymmetrical forms characterized by an escarpmentor scarp, which normally forms steep slopes of cliffs,crowned by more resistant beds, and a dip slope, whichruns along the dip of the strata. Homoclinal ridges, orstrike ridges, are only just asymmetrical and developin more steeply tilted strata with a dip between 10and 30 degrees. Hogbacks are symmetrical forms thatdevelop where the strata dip very steeply at 40 degreesplus. They are named after the Hogs Back, a ridge ofalmost vertically dipping chalk in the North Downs,England.

On a larger scale, large warps in the ground surfaceform major swells about 1,000 km across. In Africa,raised rims and major faults separate eleven basins,

134 STRUCTURE

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SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 135

Hogback Homoclinal ridge

Anti-dipstream

Dipstream

DipslopeStrike

stream

Scarpslope

Cuesta Plateau

Mesa

Butte

Figure 5.16 Landforms associated with dipping and horizontal strata cuesta, homoclinal or strike ridge, hogback,butte, mesa, and plateau. The chief streams found in landscapes with dipping strata strike streams, anti-dip streams, anddip streams are shown. Notice that a cuesta consists of a dip slope and a steeper escarpment of scarp slope. The blackband represents a hard rock formation that caps the butte, mesa, and plateau.

including the Congo basin, Sudan basin, and Karoobasin.

Folds, rivers, and drainage patterns

Geomorphologists once described individual streamsaccording to their relationship with the initial sur-face upon which they developed. A consequent streamflowed down, and was a consequence of, the slope of thepresumed original land surface. Streams that developedsubsequently along lines of weakness, such as soft strataor faults running along the strike of the rocks, were sub-sequent streams. Subsequent streams carved out newvalleys and created new slopes drained by secondaryconsequent or resequent streams, which flowed in thesame direction as the consequent stream, and obsequentstreams, which flowed in the opposite direction. Thisnomenclature is defunct, since it draws upon a presumedtime-sequence in the origin of different streams. In real-ity, the entire land area drains from the start, and it ispatently not the case that some parts remain undraineduntil main drainage channels have evolved. Modernstream nomenclature rests upon structural control ofdrainage development (Figure 5.16). In regions where asequence of strata of differing resistance is tilted, streams

commonly develop along the strike. Strike streamsgouge out strike valleys, which are separated by strikeridges. Tributaries to the strike streams enter almost atright angles. Those that run down the dip slope are dipstreams and those that run counter to the dip slopeare anti-dip streams. The length of dip and anti-dipstreams depends upon the angle of dip. Where dip is gen-tle, dip streams are longer than anti-dip streams. Wherethe dip is very steep, as in hogbacks, the dip streams andanti-dip streams will be roughly the same length, butoften the drainage density is higher on the anti-dip slopeand the contours are more crenulated because the anti-dip streams take advantage of joints in the hard stratumwhile dip streams simply run over the surface.

Most stream networks are adapted to regional slopeand geological structures, picking out the main fracturesin the underlying rocks. The high degree of conformitybetween stream networks and geological structure is evi-dent in the nine chief drainage patterns (Morisawa 1985).A tenth category, irregular or complex drainage, whichdisplays no unambiguous pattern, could be added ascould an eleventh, deranged drainage, which formson newly exposed land, such as that exposed beneatha retreating ice sheet, where there is almost no struc-tural or bedrock control and drainage is characterized

136 STRUCTURE

by irregular stream courses with short tributaries, lakes,and swamps. Figure 5.17 shows the major types ofdrainage pattern and their relationship to structuralcontrols:

1 Dendritic drainage has a spreading, tree-like pat-tern with an irregular branching of tributaries inmany directions and at almost any angle. It occursmostly on horizontal and uniformly resistant strataand unconsolidated sediments and on homogeneousigneous rocks where there are no structural controls.Pinnate drainage, which is associated with very steepslopes, is a special dendritic pattern wherein the trib-utaries are more or less parallel and join the mainstream at acute angles.

2 Parallel drainage displays regularly spaced andmore or less parallel main streams with tributariesjoining at acute angles. Parallel dip streams domi-nate the pattern. It develops where strata are uni-formly resistant and the regional slope is marked,or where there is strong structural control exertedby a series of closely spaced faults, monoclines, orisoclines.

3 Trellis drainage has a dominant drainage directionwith a secondary direction parallel to it, so that pri-mary tributaries join main streams at right angles

( ) Annulari

( ) Trellisc

( ) Rectangularh( ) Distributaryg( ) Centripetalf

( ) Centrifugale( ) Radiald( ) Parallelb( ) Dendritica

Figure 5.17 Drainage patterns controlled by structure or slope.Source: Mainly after Twidale and Campbell (1993, 342) and adapted from Twidale (2004, 173)

and secondary tributaries run parallel to the mainstreams. It is associated with alternating bands ofhard and soft dipping or folded beds or recentlydeposited and aligned glacial debris. Fold mountainstend to have trellis drainage patterns. An example isthe Appalachian Mountains, USA, where alternatingweak and strong strata have been truncated by streamerosion.

4 Radial drainage has streams flowing outwards in alldirections from a central elevated tract. It is foundon topographic domes, such as volcanic cones andother sorts of isolated conical hills. On a large scale,radial drainage networks form on rifted continentalmargins over mantle plumes, which create litho-spheric domes (Cox 1989; Kent 1991). A postulatedDeccan plume beneath India caused the growth of atopographic dome, the eastern half of which is nowgone (Figure 5.18a). Most of the rivers rise closeto the west coast and drain eastwards into the Bayof Bengal, except those in the north, which drainnorth-eastwards into the Ganges, and a few thatflow westwards or south-westwards (possibly alongfailed rift arms). Mantle plumes beneath southernBrazil and southern Africa would account for manyfeatures of the drainage patterns in those regions(Figure 5.18bc).

Ganga

Narmada

Para

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Zambezi

Cubango

Orange

Vaal

TaptiWeste

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Arabian

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OranjemondOranjemond

DurbanDurban

PretoriaPretoria

BiHighlands

KalahariDesert

Dra

kensb

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SoPaulo

Rio de JaneiroRio de Janeiro

Vitoria

KaribaLake

Limpopo

Etendeka

( )a

( )b

( )c( )c

Bay of

Bengal

Bay of

Bengal

400 km400 km 400 km400 km

400 km400 km

Figure 5.18 Drainage patterns influenced by mantle plumes. (a) The drainage pattern of peninsula, India with thepostulated Deccan plume superimposed. Most of the peninsula preserves dome-flank drainage. The Gulf of Cambay,Narmada, and Tapti systems exhibit rift-related drainage. (b) The drainage pattern of southern Brazil with superimposedplume. Dome-flank drainage is dominant except near Porto Alegre. (c) The drainage pattern in south-eastern andsouth-western Africa with the Paran plume (left) and Karoo plume (right) superimposed. Rivers over the Paran plumeshow an irregular dome-flank pattern drainage eastwards into the Kalahari. Notice that the Orange River gorge is formedwhere antecedent drainage has cut through younger uplift. Rivers over the Karoo plume display preserved dome-flankdrainage west of the Drakensberg escarpment. The dotted line separates dome-flank drainage in the south fromrift-related drainage in the north.Source: Adapted from Cox (1989)