Reddish-brown to brown clayey silt, silty sand to sandy silt, and medium to coarse quartz sand with pebbles (Ramsey, 2005). Rock fragments of mica or sillimanite quartzose schist are common sand fraction. At land surface, a gray to grayish-brown clayey silt is present. Sands are cross-bedded with laminae of muscovite or heavy minerals defining the cross-sets. Silty beds tend to be structureless, or in the gray clayey silt beds, heavily bioturbated by roots. No fossils other than pollen have been recovered. Pollen indicate a cold climate during deposition of the upper clayey silt unit (unpublished DGS data). Stratigraphic relationships indicate either slightly younger than or contemporaneous with the Columbia Formation. Ranges from 5 to 40 ft in thickness.
This publication formally establishes the Old College Formation, a lithostratigraphic unit located along the Fall Zone of Delaware. It is named for sediments encountered in numerous drill holes on, and adjacent to, the Old College campus of the University of Delaware in Newark, Delaware. The Old College Formation consists of micaceous, brown to reddish-brown, fine to coarse sand with scattered gravelly sand overlain by sandy silt beds. The Old College Formation has a distinctive suite of abundant heavy minerals including sillimanite, staurolite, and magnetite. Provenance of the sand is local, derived from erosion of Piedmont rocks along and just to the west of the Fall Zone. The unit is the result of alluvial fan deposition on a pediment-like surface extending from the Fall Zone to the adjacent Coastal Plain. The Old College Formation is a surficial unit that overlies Piedmont saprolite, the Cretaceous Potomac Formation, and the Pleistocene Columbia Formation. No fossil data are available for the unit. Stratigraphic and geomorphic positions indicate that it ranges from 500,000 to 1,000,000 years old; slightly younger than the Columbia Formation.
This map shows the surficial geology of New Castle County, Delaware at a scale of 1:100,000. Maps at this scale are useful for viewing the general geologic framework on a county-wide basis, determining the geology of watersheds, and recognizing the relationship of geology to regional or county-wide environmental or land-use issues. This map, when combined with the subsurface geologic information, provides a basis for locating water supplies, mapping ground-water recharge areas, and protecting ground and surface water. Geologic maps are also used to identify geologic hazards, such as sinkholes and flood-prone areas, to identify sand and gravel resources, and for supporting state, county, and local land-use and planning decisions.
This dataset contains the geologic polygons used for the creation of DGS Geologic Map 13. This dataset shows the surficial geology of New Castle County, Delaware, at a scale of 1:100,000.