Geologic Formations

Delaware Piedmont

The Delaware Piedmont is a portion of the Appalachian Mountains. This area was created as a result of tectonic activity that occurred during the Paleozoic era, between 543 and 245 million years ago. During the convergence of the North American plate approximately 470 to 440 million years ago deformed and metamorphosed rocks were created. Rocks that are formed within the deep basin are called the Wissahickon Formation which is what makes up most of the underlying rocks found throughout the Brandywine Valley Unit.

Wissahickon Formation

• Found in Pennsylvania, Delaware and New Jersey

• Identifiable as steep slopes and sharp valleys

• Composed of gneiss, amphibolite, pegmatite and serpentinite

Atlantic Coastal Plain

• Found in Pennsylvania, Delaware, New Jersey, Massachusetts, Rhode Island, New York, Maryland, the District of Columbia, Virginia, North Carolina, South Carolina, Georgia, and Florida

• Identifiable as clay, marine silt and sands

• Composed of silt, sand, and gravel

Delaware’s Atlantic Coastal Plain

The Atlantic Coastal Plain sediment in Delaware is approximately 8,000 feet thick. The formation process began in the early Cretaceous period, approximately 120 million years ago, when the waterways between here and the Appalachians (north of Delaware) transported clay into southern Delaware. This formation was called the Potomac Formation.

After a period of non-deposition, the Magothy Formation began depositing marine sediments on top of the Potomac Formation layer. This formation began during the Cenozoic period, approximately 65 million years ago through to present day. Deposition of marine sediments has occurred multiple times and forms the aquifer that provides water to southern Delaware. In the last 12,000 years, the sea level along the coast has been rising moving the shoreline inward.

Potomac Formation

• Fluvial (river) processes

• 20 - 1600 feet thick

• Composed of silty clay to clayey silt in various colors

Magothy Formation

• Coastal to nearshore environments; Marine processes

• 20 – 50 feet thick

• Composed of dark grey to grey, silty clay to clayey silt, with fragments of lignite

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References

• Commonwealth of Pennsylvania. (1988, December 5). Official State Fossil - Phacops rana Act of Dec. 5, 1988, P.L. 1113, No. 138. Retrieved November 2019, from Pennsylvania General Assembly: http://www.legis.state.pa.us
• Coxe, R. (2014). Historical Analysis and Map of Vegetation Communities, Land Covers, and Habitats of the Woodlawn Tract New Castle County, Delaware. Department of Natural Resources and Environmental Control, Division of Fish and Wildlife. Smyrna, DE: DNREC. Retrieved 2019
• Delaware Geological Survey. (2019). The Geology of Delaware. Retrieved from The Delaware Geological Survey: Geologic and hydrologic research and exploration for Delaware: https://www.dgs.udel.edu
• Delaware Geological Survey. (2019). The Geology of Delaware. Retrieved from Geologic and hydrologic research and exploration for Delaware: https://www.dgs.udel.edu
• Plank, M. O., & Schenck, W. S. (1998). Delaware Piedmont Geology (Special Publication No. 20 ed.). Newark, DE: Delaware Geological Survey University of Delaware. Retrieved from https://www.dgs.udel.edu/sites/default/files/publications/sp20.pdf
• US Geological Survey. (2016, September 30). Atlantic Coastal Plain, Maryland to Florida. Retrieved November 2019, from US Geological Survey: www.usgs.gov