Washington, D.C., is the first and largest planned city in the United States. The city lies along the Fall Line at the boundary between the Atlantic Coastal Plain and the Piedmont Plateau and at the head of navigation on the estuary of the Potomac River. This site combines the engineering complexities of two vastly different geologic terranes with the other complications introduced by the terraces and channels of a major river-estuary system.
The western part of the city and most of the suburbs to the west and north are on the Piedmont Plateau, an upland underlain by complexly deformed metasedimen-tary and metaigneous rocks of late Precambrian or early Paleozoic age. These crystalline rocks are mantled by soil, saprolite, and weathered rock to depths of as much as 50 m, which adds both to their geologic inscrutability and to the problems of excavation and design of structures.
The Atlantic Coastal Plain is underlain by unmetamorphosed and little deformed fluvial and marine strata of Cretaceous through Miocene age. These deposits form a prism that thickens southeastward from a wedge edge at the Fall Line to as much as 450 m in the southeastern part of the metropolitan area. Unconformities, facies changes, and variations in physical properties with age and depth of burial add spice to the life of the engineering geologist dealing with these strata.
Terrace deposits ranging in age from Miocene(?) to Holocene bevel across the contact between the Coastal Plain deposits and the crystalline rocks of the Piedmont. The oldest deposits underlie a broad, deeply dissected upland that stands at an elevation of 80 to 90 m southeast of the Fall Line; isolated outliers cap hills and interfluves at elevations of as much as 150 m northwest of the Fall Line. Lower and younger terraces flank the major drainages and occur at various levels down to the modern flood plains. Much of the central city is built on low terraces of Sangamon or Wisconsin age. These younger terraces locally fill and conceal deep bedrock channels cut by the ancestral Potomac during low stands of sea level during the Pleistocene. The terrace deposits show conspicuous differences in degree of weathering and soil development, depending on their age and physiographic position. Estuarine and marsh deposits flank the tidal reaches of the Potomac and Anacostia Rivers, and considerable parts of the central city are built on artificial fill over these deposits.
Considerable experience in underground excavation has been gained in the last decade during construction of METRO, a regional rapid transit rail system. Tunneling techniques have been developed for both crystalline rocks and Coastal Plain deposits, but cut and cover methods are generally used in the young materials, which are generally weakest. Foundation and slope stability problems are widespread in some geologic units in the metropolitan area and are locally serious. They affect structures ranging from single family dwellings to the Washington Monument.
