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Configuration of the bedrock surface of the District of Columbia and vicinity

1950; United States Government Publishing Office; Linguagem: Inglês

10.3133/pp217

2330-7102

Nelson Horatio Darton,

Botany, Ecology, and Taxonomy Studies

A bedrock floor of granite, gneiss, schist, and other crystalline rocks underlies the District of Columbia region. It is at or near the surface in the vestern part and descends gradually eastward below the wedge-s:mped mass of overlying sedimentary formations of the Coastal Plain. Much of Washington is built across this overlap of unconsolidated material on crystalline rock. In the valley of Rock Creek and in the rolling lands to the west the crystalline rocks crop out, but east of Rock Creek the bedrock surface slopes steadily southeastward, and in the eastern part of the city it lies several hundred feet below the surface. The position of the bedrock is a matter of great interest in planning foundations for governmental and other large buildings, and the relations of the basal gravels of the Coastal Plain sediments are important because they contain water supplies, which have been utilized commercially for condensing and other purposes. Data as to bedrock configuration are difficult to obtain because in drilling wells and sinking boreholes the materials are not always identified accurately. At the outcrop, and to some extent underground, much of the bedrock is decayed to a micaceous sand that is not everywhere easy to distinguish from sedimentary material. Data from nearby holes help in verification from one to the other. The bedrock surface, described in this report and shown on the maps and sections, is the geologic contact between the bedrock formations and overlying unconsolidated materials of younger sedimentary formations. This surface rarely coincides with the top of hard rock due to the weathering of the bedrock formations to irregular depths. Although the southeasterly slope of the bedrock is in general very regular at a rate of 100-150 feet to the mile, there are local variations in direction and amount. Where data are closely spaced they reveal low mounds and ridges and shallow valleys which had been developed as surface features on the old pre-Cretaceous rocks upon which the beds of the Potomac group were deposited. Possibly also some faulting or even flexing has taken place, which would cause irregularities in the bedrock floor These local irregularities are described in as much detail as data permit and are illustrated by a number of maps and sections. The boreholes that afford a basis for the altitude lines in the maps and diagrams are about 600 in number, but not all of them were correctly interpreted by the drillers as to materials penetrated or identity of bedrock.