The Rock Creek area contains some spectacular examples of geological features, and the geology has influenced land use and water flow throughout the area’s history.
Along the length of the stream there is a boundary between two of the eastern United States' large geological provinces, the Piedmont Province and the Coastal Plain Province. The boundary between the two is marked by the Fall Line, the watershed’s most prominent geological feature.
Evidence of the Fall Line can best be seen along a steep segment of Rock Creek between Military Road and Tilden Street
The area to the north and west is part of the Piedmont Plateau and has exposed metamorphic rocks, hilly to rolling terrain, and fast flowing streams. This means that the entire Montgomery County section of Rock Creek is part of the Piedmont Plateau. The predominant rocks in this area include boulder gneiss, mica schist, and quartzite. To the east and south lays the Coastal Plain with its flatter terrains, few rocky outcrops, younger poorly consolidated sediments, and meandering streams. This area contains mostly fluviatile, channel fill, sand, gravel, and some silt and clay. For a detailed map of the geology of Rock Creek Park (courtesy of the National Park Service) click here.
Within Rock Creek Park there are numerous quarry sections where, in the past, rocky materials were taken for building materials. The Conecticut Avenue corridor once claimed as many as 22 active quarries. More than 10 major quarry sites are still visible. One of the most easily accessible of these is the Broad Branch or Peirce quarry site on Broad Branch Road. The rock is Kensington gneiss, which is a black and white highly jointed rock.
Use of this stone can be seen in many buildings in northwest Washington, DC, including the "Think Tank" building at the National Zoo. The Peirce Quarry site was incorporated into the park upon its creation in 1890, but not officially purchased until the 1900s. Until this time the quarry continued to operate. This site is currently used as a scientific lab and for rapelling training.
The Rock Creek Park Nature Center provides information on several interesting geological sites in the National Park, with directions to each, and has more detailed reports on the geology of the region.