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 is a map of the crystalline bedrock units in the Piedmont of Delaware and adjacent Pennsylvania. The southern boundary of the mapped area is the updip limit of the Potomac Formation (Woodruff and Thompson, 1972, 1975). Soil, regolith, and surficial deposits of Quaternary age are not shown.
The quantitative lithofacies analysis of the Potomac Formation in a small area west of Delaware City revealed that the deposition of these sediments was continuous throughout the time of their formation. The uppermost part of the Potomac sequence appears to have been removed, probably by erosion, prior to the deposition of the younger Upper Cretaceous marine sediments. The sand bodies contained in Potomac deposits have a shoestring channel form and were most probably deposited by unidirectional currents. The direction of the flows, however, cannot be determined on the basis of the available subsurface data.
Recent erosion along the Chesapeake and Delaware Canal has exposed an unusually rich Upper Cretaceous fossiliferous outcrop at the Biggs Farm, near the eastern end of the Canal. Some III species of mollusks representing 72 genera have been identified. Coelenterata, Porifera, Annelida, Brachiopoda, Crustacea, and a few fragmentary vertebrate remains have also been found. Five species are being described as new, and there are 54 new records for the Cretaceous of Delaware.
The preservation of the material suggests that the animals lived on a sandy bottom in water between 50 and 100 feet in depth, possibly near the mouth of a bay.
Inasmuch as there is a mixing of some species characteristic of the Matawan Group and other species characteristic of the Monmouth Group, it is believed that the fauna at this locality lies near the Matawan-Monmouth boundary, perhaps in the lower part of the Monmouth Group.