The Geology of Washington, D.C., is broadly divisible into two regions. The northwestern quadrant of the city lies mainly in the Appalachian

Piedmont it, Piemontese , population_note = , population_blank1_title = , population_blank1 = , demographics_type1 = , demographics1_footnotes = , demographics1_title1 = , demographics1_info1 = , demographics1_title2 ...

region, marked by moderate to steep hills underlain by

metamorphic rocks Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, cau ...

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. ...

through

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, w ...

age, similar to the adjacent Piedmont regions of

Montgomery County, Maryland Montgomery County is the most populous county in the state of Maryland. As of the 2020 census, the county's population was 1,062,061, increasing by 9.3% from 2010. The county seat and largest municipality is Rockville, although the census-design ...

. The

National Mall The National Mall is a landscaped park near the downtown area of Washington, D.C., the capital city of the United States. It contains and borders a number of museums of the Smithsonian Institution, art galleries, cultural institutions, and va ...

, northeastern and southeastern quadrants of Washington lie in the mid-Atlantic

Coastal Plain A coastal plain is flat, low-lying land adjacent to a sea coast. A fall line commonly marks the border between a coastal plain and a piedmont area. Some of the largest coastal plains are in Alaska and the southeastern United States. The Gulf Co ...

region, consisting of

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

deposits laid down during the much more recent

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non- avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

and Quaternary periods, like the bordering areas of southern Maryland. Some areas in Northeast and Southeast Washington also feature hilly terrain, especially east of the

Anacostia River The Anacostia River is a river in the Mid Atlantic region of the United States. It flows from Prince George's County in Maryland into Washington, D.C., where it joins with the Washington Channel to empty into the Potomac River at Buzzard Poi ...

; this however stems from rivers and creeks down-cutting into the relatively soft soil. City founders George Washington and

Pierre L'Enfant Pierre "Peter" Charles L'Enfant (; August 2, 1754June 14, 1825) was a French-American military engineer who designed the basic plan for Washington, D.C. (capital city of the United States) known today as the L'Enfant Plan (1791). Early life a ...

chose to locate the District of Columbia on the

fall line A fall line (or fall zone) is the area where an upland region and a coastal plain meet and is typically prominent where rivers cross it, with resulting rapids or waterfalls. The uplands are relatively hard crystalline basement rock, and the coas ...

of the

Potomac River The Potomac River () drains the Mid-Atlantic (United States), Mid-Atlantic United States, flowing from the Potomac Highlands of West Virginia, Potomac Highlands into Chesapeake Bay. It is long,U.S. Geological Survey. National Hydrography Datas ...

, the point furthest inland reachable by ships coming up the river. (See also Atlantic Seaboard fall line.) Below

Roosevelt Island Roosevelt Island is an island in New York City's East River, within the borough of Manhattan. It lies between Manhattan Island to the west, and the borough of Queens, on Long Island, to the east. Running from the equivalent of East 46th to 8 ...

, the Potomac spills out onto the Coastal Plain, leaving the rock formations of Great Falls and the rocky cliffs of the

Arlington, Virginia Arlington County is a county in the Commonwealth of Virginia. The county is situated in Northern Virginia on the southwestern bank of the Potomac River directly across from the District of Columbia, of which it was once a part. The county i ...

shore behind as it slows and widens. The Tidal Basin and the Washington Channel are part of this wider section of the river. Much of the National Mall was

marsh A marsh is a wetland that is dominated by herbaceous rather than woody plant species.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p Marshes can often be found a ...

in the early Nineteenth century; as the capital city grew, the marsh was filled in and reclaimed.

References

{{Geology of the United States by political division Washington, D.C. Natural history of Washington, D.C.