The Nanjemoy Formation is a geologic formation pertaining to both the Wilcox Group and the Pamunkey Group of the eastern United States, stretching across the states of Virginia, Maryland, and District of Columbia. The formation crops out east of the Appalachians and dates back to the Paleogene period. Specifically to the Ypresian stage of the Eocene epoch, about 55 to 50 Ma or Wasatchian in the NALMA classification, defined by the contemporaneous Wasatch Formation of the Pacific US coast.
The about thin formation crops out in a narrow irregular band and only in certain of the many creeks of the Maryland peninsula and on the southern side of the Potomac River in Virginia. The two members the formation was divided into by Clark and Martin in 1901; Potapaco and Woodstock, represent different phases in the basin history. The lower Potapaco Member is much more clayey, described as marl, than the upper Woodstock Member, probably characteristic of less storm influences in the shallow shelf sediments.
The formation has provided a wealth of fossils of mainly fish, but also mammals, reptiles, birds and flora. The presence of the sharks Otodus obliquus and Carcharocles aksuaticus, as well as various other shark and ray species are notable. Crocodylian, snake, turtle, mammal, and bird remains have all been found in the Nanjemoy Formation.
Etymology
Nanjemoy is probably an Ojibwe word, meaning "one goes downward", representing the many rivers and creeks in the wet watershed of Chesapeake Bay, an area originally inhabited by the Algonquin-speaking Nanticoke and Powhatan. The formation is named after Nanjemoy Creek, a left tributary of the Potomac River.
Wicomico is the name of two separate rivers, one on Maryland's Eastern Shore, Wicomico River (Maryland eastern shore), and another in Southern Maryland, Wicomico River (Potomac River tributary), and is the name of a small thoroughfare in southeastern Charles County.
The reason this term appears in three different places in the state simply means that the people living there when the English barged in were trying to tell them it was a nice place to live.
Wigh or wik, representing "pleasant" and accomico "dwelling, village" was the native's way of telling the Europeans they liked living there.
Potapaco was an early name for Port Tobacco Creek, that was named after the people, also known as the Piscataway.
Nanjemoy, which may contain elements of Ojibwa, by contrast means "one goes downward."
Mattawoman, found in Charles, Prince George's and on the Eastern Shore, has remnants of not only Algonquin, but Fox and Ojibwa as well, and translates to "where one goes pleasantly."
Definition
The Nanjemoy Formation was defined by Clark and Martin in 1901, as part of the mapping by the Maryland Geological Survey. The Nanjemoy Formation was divided into two members, the lower or Potapaco, and the upper or Woodstock. The main lithologic distinction is that the lower part of the Nanjemoy is much clayey than the upper part. In the subsurface, the distinction between the members is less evident than in outcrops, especially in Maryland, so the formation has been left undivided.
In the outcrops along the Potomac River near Popes Creek, the contact between the Woodstock and Potapaco at about above the water level.
Extent
The Nanjemoy Formation, represented as Tn on the geologic map of the Washington West 30' × 60' Quadrangle, Maryland, Virginia, and Washington D.C., restricted to the southeastern side of the Potomac Basin represented in the quadrangle. Scott (2005) in his thesis included a map showing the thin bands of outcrops of both the underlying Marlboro Clay and the Nanjemoy Formation, restricted to the many creeks feeding the Chesapeake Bay.
The Nanjemoy Formation (Eocene), the Marlboro Clay (Paleocene), and the Aquia Formation (Paleocene) are present in the westernmost part of the Potomac channel.
The outcrop area of the formation is designated Nanjemoy Wildlife Management Area.
Geology
Geologically, the area of deposition of the Nanjemoy Formation is part of the Atlantic coastal plain province.