Calvert Cliffs Fossils
by Jeanne D. McLennan
The Calvert Cliffs on the western shore of the Chesapeake Bay in Calvert County are justly famous as a fossil collecting area. The fossiliferous deposits belong to the Chesapeake Group of Miocene age geological strata in the Atlantic Coastal Plain region. These deposits are exposed in cliffs up to 100 feet high between Chesapeake Beach and Drum Point and constitute the most complete section of Miocene deposits in the eastern United States.
The Miocene was a period of uplift in Middle America and the Antillean region, that was accompanied by folding of the crust of the earth and volcanism. Erosion of the recently uplifted areas produced extensive deposits of clay, sand and marl. Some of these sediments were consolidated, forming shales and sandstone. During this time, North and South America were united, and the island of Florida joined the Georgia mainland. The North American continent assumed approximately its present outlines.
The Chesapeake Group is divided into three formations. The oldest, the Calvert Formation, is composed of diatomaceous earth and dark sandy clays and marl. It is overlain by the yellowish sand and greenish clay and marl of the Choptank formation. The youngest deposits belong to the St. Marys Formation, which, as exposed at Little Cove Point, consists of bluish sandy clay and fine sandstone. The silty and sandy content of the Miocene deposits in Maryland was derived from the erosion of older Coastal Plain deposits and crystalline rocks of the Piedmont region. The calcareous deposits are organic in origin.
The first fossil described from North America was found in deposits of the St. Marys Formation. This fossil, Ecphora quadricostata, shown on the cover, was illustrated in a work on Mollusca published in England in 1685.
At the base of the Calvert Formation are varying thicknesses of gray to white diatomaceous earth which formed from the siliceous tests (coverings) of myriad microscopic aquatic plants called diatoms. In the deposits above the diatomaceous earth is a unique assemblage of fossils. Representatives of nearly every animal phylum have been identified here. Of the 624 species identified, 408 are of Mollusca.
Whales are the most abundant marine vertebrates, and are found associated with porpoises, dolphins, and sea cows. Sharks and rays were plentiful as evidenced by the number of teeth and dental plates found. The birds are represented by the booby, gannet, fulmar, and shearwater, all of which may be found along sea coasts today. Remains of fresh water and marine turtles and land tortoises also occur. Crocodile teeth may be found occasionally, so perhaps these animals made their nests in the sand of the Calvert Formation.
Of the Mollusca, gastropods and pelecypods are the most numerous, but cephalopods are rare. Only one species of brachipod has been found. Echinoderms, crabs, and barnacles are represented, and worm boreholes also occur.
Some tentative conclusions on the environments in which these animals lived may be drawn from the assemblage of fossils in the Chesapeake Group. From the two species of corals found it is considered that the sea was too cool for reef corals and too shallow for deep-sea forms. On the other hand, the presence of whales and porpoises indicates that there was free access to the open sea. Evidence suggests that during the Miocene Period the Chesapeake Embayment was a shallow, temperate sea that covered Southern Maryland. It was bordered by low sandy shores, tidal marshes, and fresh water swamps with bald cypress. There are indications that a progressive but slight cooling of the temperature occurred from Calvert to St. Marys time. In the Pliocene that followed, there was a rise in temperature.
The skeletal remains of such land fauna as tapirs, mastodons, rhinoceros, horses, and dogs are sometimes found here. The occurrence of these animals may be accounted for in several ways. It is quite probable that the embayment was fed by fresh water streams and rivers; therefore during floods or seasonal rains these animals could have been swept downstream from their natural habitat. On the other hand, the animals may have been mired in the swamps or trapped at the foot of banks by high tides. The marine mammal remains are generally disarticulated. Unless the carcasses were covered rapidly by sediment, they would have been eaten by predators, or the bones would have been scattered by surf or by tidal scour.
The public beaches on the shore of Calvert County are a rewarding area for collectors of well-preserved fossil shells and sharks teeth. Sometimes small, very round holes will be noted in the shells. These were caused by borers that attacked the living Mollusca, and are not man-made. In general, the fossil shells may be distinguished from present day forms by the fact that their shells are usually thicker and are chalky white or gray. Modern shells are more colorful and often are glossy.
The most comprehensive guide for the identification of specimens is the book Miocene Plates, the companion volume to the report Miocene Text. A smaller and simplified version of these two books, Miocene Fossils of Maryland, by Harold E. Vokes is useful. All of these are publications of the Maryland Geological Survey.
Collecting at the base of the high cliffs is not recommended. Access
to the area is difficult, and most of the land is privately owned. In
addition, large blocks often fall from the overhanging cliffs and can
cause serious accidents.
Contact our publications office for an interactive BULLETIN 20: Miocene Fossils of Maryland on CD-ROM
Clark, W.B., and others, 1904, Miocene Text: Maryland Geological Survey Systematic Report, 509 pp.
Clark, W.B., and others, 1904, Miocene Plates: Maryland Geological Survey, Systematic Report, 135 plates.
Gernant, R.E., Gibson, T.G., and Whitmore, F.C., 1971, Environmental History of Maryland Miocene: Maryland Geological Survey Guidebook No. 3, 58 pp.