Specifically to the Ypresian stage of the Early 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 20 metres (66 ft) 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.

[2] Nanjemoy is probably an Ojibwe word, meaning "one goes downward,"[2] representing the many rivers and creeks in the wet watershed of Chesapeake Bay, an area originally inhabited by the Algonquin-speaking Nanticoke and Powhatan.

[6] 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.

It includes all of the land area where water drains towards the mouth of the Potomac – the point where the river spills into the Chesapeake Bay.

Both geologic groups have been dated to the early Paleogene; the Paleocene and Eocene periods, or in the commonly used NALMA classification; Wasatchian, defined by the age-equivalent Wasatch Formation of Wyoming.

The unit is present only in the southeastern part of the map area of Washington D.C., and it reaches a maximum thickness of about 20 metres (66 ft).

[6] The formation comprises glauconitic quartz sand, dark-grayish-green to olive-black (tan to orange where weathered), fine to medium-grained; and dark-greenish-gray silty clay.

[6] Robert E. Weems and Gary J. Grimsley (1999) described the geology of the Fisher/Sullivan site in Virginia as:[10] The climate of the Early Eocene and Late Paleocene was very hot and rich in CO2.

Based on the fossils of the terrifyingly large Titanoboa cerrejonensis, dating to the latest Paleocene, a body mass of 1,135 kilograms (40,000 oz) for the snake was estimated.

This enormous size could only have been possible under the most ideal conditions for such large species, around 6 to 8 Ma after the extinction of the previous megafauna; the dinosaurs.

[5] The floral and faunal assemblage of the Nanjemoy Formation is very varied and provides an insight into the paleobiological and paleoclimatological environment of the early Eocene.

Fossils of bivalves, sharks, rays, actinopterygian fishes, reptiles, birds and mammals, and of fruits and seeds are common in the Potapaco Member.

[20] Brezina immediately realized that the site was exceptional, because it yielded numerous shark teeth and other vertebrate remains from the sands and gravels in the unnamed tributary.

It soon became apparent, from the types of teeth that were being found and from the color and texture of the sediments in the banks of the creek, that the fossils were being reworked from glauconitic ("greensand") horizons of the Lower Tertiary (Paleocene-Eocene) Pamunkey Group.

[20] Because the Pamunkey Group previously had yielded only sparse vertebrate remains, it seemed reasonable to suspect that this locality was scientifically important.