Geologic History of the Delaware Coastal Plain

Delaware lies within two physiographic provinces, the Appalachian Piedmont and the Atlantic Coastal Plain. The Fall Zone divides these two provinces. The major events in the evolution of the Piedmont rocks occurred between 500 and 200 million years ago.

A great period of time, of which there is no record in Delaware, passed before the deposition of the oldest sediments of the Coastal Plain, the Potomac Formation, during the latter part of Early Cretaceous time, about 120 million years ago. Streams transported clays with interbedded sands from the Appalachian mountains which lay to the northwest and deposited them in rivers and swamps. This process continued into the Late Cretaceous resulting in a wedge of sediments with a thickness of about 4,000 feet in southern Delaware. During the Late Cretaceous, between 100 million and 65 million years ago, sea level rose, and marine sediments were deposited across Delaware. The sands of the Potomac Formation form the Potomac aquifer which provides water to residents of New Castle County.

A small unconformity, or period of nondeposition, separates the Potomac Formation from the overlying Magothy Formation. The white sands and lignitic black silts of the Magothy form a distinctive marker indicating the transition from the older sediments to the later marine deposits.

During the Cenozoic, between 65 million years ago and the present, sea level rose and fell multiple times depositing coastal and marine sediments across a large area of the Delaware Coastal Plain. In Delaware, these deposits are called the Rancocas and Chesapeake Groups. These large stratigraphic groups are subdivided into many formations. The sands in these formations form aquifers which provide water for southern Delaware.

The most recent phase of deposition occurred about 2.4 million years ago when glaciers advanced and retreated across the northern part of North America during the early Pleistocene period. The Beaverdam Formation, which is found over much of southern Delaware, is the result of climate change which transported much of the weathered soils and rocks of the Appalachian Piedmont toward the ocean. The Columbia Formation, which is found in the northern part of the Delaware Coastal Plain, consists primarily of sand with gravel generated by the first glaciers that advanced into the Delaware and Susquehanna River basins. The Beaverdam and Columbia Formations comprise the surficial Columbia aquifer, which provides much of the water for irrigation and domestic wells throughout Kent and Sussex counties.

During the middle to late Pleistocene, sea level rose during warm periods and receded during glacial periods. When sea level was high, (higher than present sea level), sediments were deposited along the margins of the Atlantic Ocean and the Delaware River in environments much like those found along the coast today. During the times when the glaciers were at their maximum in central Pennsylvania,northern New Jersey, and Delaware experienced very cold and windy climates which resulted in the formation of permafrost features and sand dunes across much of its area. Sea level has been rising over the last 12,000 years, advancing the shoreline landward.

Total thickness of all coastal plain units is about 8,000 feet (at Fenwick Island).