Delaware Reserve

St. Jones River - Delaware

Geology

The St. Jones River component is within the Coastal Plain Province approximately 45 miles south of the Appalachian Piedmont Fall Zone. The Piedmont-type rocks are covered by a thick wedge of unconsolidated and semiconsolidated sedimentary rocks. The oldest and most extensive of these sediments are at the base of the Potomac Formation and are about 120 million years old. It consists of color-banded clays with interbedded sands that eroded off the ancestral Appalachian Mountains.

The Magothy Formation was deposited next with its very distinct white sands and black lignite suggesting a transitional environment from stream deposits to marine, much like that found in a delta. Layered on top of the Magothy are marine formations of Cretaceous through Eocene age with the Piney Point Formation being the youngest. Above this is a nonconformity, which represents a gap in the sedimentary record during which no sediments have been preserved (Oligocene age.)

Later, the sea again covered most of Delaware and deposited Chesapeake Group (Miocene age). This group consists of interbedded silts and sands and reaches a thickness of 400 feet at the St. Jones. Many of the sandy layers contain important supplies of water for municipal and industrial use in the Dover area. The repeated advance and retreat of continental glaciers during the past one to two million years (Pleistocene age) caused dramatic changes in relative sea level and the configuration of streams draining from the glaciers. The deposits from meltwater runoff supplies most of the sands and gravel for construction. Sand and gravel are the most important mineral resources in Delaware with the most potential source for Kent County being in and around the St. Jones River Component area.