Geology of the Washington, D.C. area

IMPACT! – A Geological Formation Overview

• I.M.P.A.C.T. = “Imagine the Movement of Plates that created both the Appalachian Mountains and the Atlantic Ocean basin along with the Crater from the Chesapeake Bay meteorite that helped shape the Washington, D.C. area over Time”

You know the I.M.P., so let’s get to the A.C.T.

• Grenville orogeny (Precambrian)• Taconic orogeny (Late Ordovician)• Acadian orogeny (Late Devonian)• Alleghenian orogeny

(Mississippian/Pennsylvanian)• Creation of Atlantic Ocean upon

breakup of Pangea (180 Ma)

“A” = Formation of the Appalachians and Atlantic Ocean

Chesapeake Bay Meteorite and Bay Formation

“C” = Crater from the Chesapeake Bay Meteorite

• Exmore Crater result of impact from meteorite 35 million years ago

• Equivalent to the size of Rhode Island and the depth of the Grand Canyon

• Caused disruption of natural river flows, creating deep canyons in soft Coastal Plain sediment

• Freshwater from the melting of glaciers and associated flooding from the end of the last Ice Age mixed with saline Atlantic Ocean water, creating Chesapeake Bay

• Largest estuary in U.S. and assumed present shape ~3,000 years ago “T” = Geologic Time

“Geology is the study of pressure and time. That’s all it takes really, pressure, and time.” – Vance Brown

Washington, D.C.’s Geologic Setting• Atlantic Coastal Plain

• Flat, low-lying area composed of crystalline rock covered by wedge-shaped layers of sand, gravel, and clay

• Piedmont Plateau• Diverse topography composed of

metamorphosed sedimentary and volcanic rocks like schist, granite, and sandstones

• Triassic Lowland• Composed of shales and

sandstones with alluvial fans of limestone and quartz on western edge

• Blue Ridge• Region of valleys and ridges

underlain by folded metamorphic and igneous rocks like granite and greenstone

The Terraces of Washington, D.C.

The Fall Line“...we were intercepted with great craggy stones that [stand] in midst of the river, where the water falleth so rudely and with such a violence as not any boat could possibly pass...” – John Smith (1607)

• Line of rapids and falls marking where the waters rushing over the Piedmont encounter the easily erodible sediments of the Coastal Plain

Teddy Roosevelt Island

Great Falls Park• Located along the Fall Line, 14 miles upstream from Teddy Roosevelt Island• Potomac River experiences its most dramatic elevation change here, cutting

through the Piedmont and reaching the Coastal Plain• When hiking, this change can be seen first hand and it is stunning

Goes from this … … to this in less than a mile.

Rock Creek Park• Also located along the Fall Line, it covers over 2,800 acres (7% of D.C.)• Northern and western portions home to exposed metamorphic rock like

boulder gneiss, mica schist, and quartzite• Southern and eastern portions home to sand, gravel, and silt closely

associated with the Coastal Plain• Home to two rock formations, the Laurel Formation and the Sykesville

Formation, which are separated by the 1.8-mile wide system of geologic faults known as the Rock Creek Shear Zone (evidence of which in the Laurel Formation is shown in the figure on the right)

Smithsonian National Zoological Park“A geologists paradise” – R.S. Bassler, 1933

“The Zoo’s foundation is a living physical geology text book” – UDC geologists

• U-shaped bend of Rock Creek near Zoo due to faulting (shown above)

• Evidence of faulting outside Zoo (shown on right)

Darton’s Fault• Named after N. H. Darton, who

had cage put around fault in the 1920s for its protection

• Evidence of previous thrusting event

• The old metamorphic rocks of the Piedmont were pushed over the younger Coastal Plain sediments

The Building Stones of D.C. and a Brief History of the National Mall

Old stones from the CapitolKensington tonalite Red sandstone

Fun with Maps!

1976 version 2016 version

Thank You!