The Nanjemoy Formation is an Early Eocene geologic unit exposed east of the Appalachians in Virginia, Maryland, and the District of Columbia, dating to about 55 to 50 million years ago. Named for Nanjemoy Creek and defined in 1901 by Clark and Martin, it includes the lower Potapaco and upper Woodstock members. Formed in shallow-shelf conditions on the Atlantic Coastal Plain, its sands and clays record changing water energy and gentle eastward dip. It is especially important for its fossils, including abundant fish remains, sharks and rays, as well as reptiles, birds, mammals, mollusks, fruits, seeds, and numerous vertebrate coprolites, making it a key source for understanding early Eocene coastal ecosystems.