Geology and mineral resources of Southern Maryland
1971, Glaser, J.D.
Report of Investigations 15
Southern Maryland, defined here to include all of Charles, St. Marys, and Calvert as well as portions of southern Prince Georges and Anne Arundel Counties, takes in some 127 square miles of the Coastal Plain province of eastern Maryland. The area is bounded on the east by Chesapeake Bay and on the west and south by the Potomac River. Just to the north lies the burgeoning BaltimoreWashington metropolitan area.
Southern Maryland is wholly underlain by unconsolidated sediments ranging in age from Cretaceous to Pleistocene. The oldest rocks in the area are fine-grained sand, silt, and variegated clay of the Lower Cretaceous Patapsco Formation, which crops out in a narrow interrupted band in the extreme northwest. Overlying the Patapsco is the equally restricted Upper Cretaceous Monmouth Formation-mostly dark-gray muddy fossiliferous silt totaling about 30 feet of sediment. The Monmouth is succeeded unconformably by a relatively thick, tripartite Paleocene-Eocene section- the Aquia and Nanjemoy Formations separated by the Marlboro Clay-which outcrops over roughly the northwestern third of Southern Maryland. Both the Aquia and Nanjemoy are variably muddy, fossiliferous greensands in contrast to the Marlboro which is a thin but persistent pinkish to gray plastic clay. The Paleocene-Eocene section includes about 500 ft. of sediment. Miocene sediments belonging to the Chesapeake Group crop out over most of the remaining area of Southern Maryland. These include the Calvert, Choptank, and St. Marys Formations, comprising up to 400 ft. of fine dark muddy sand, silt, clay, and diatomite. Virtually all of the upland areas (greater than 100 ft. elev.) of Southern Maryland are thinly veneered with Plio-Pleistocene sand, gravel, and subordinate silt-clay, collectively termed the Upland Deposits. Finally, the youngest sediments in the survey area include the alluvium filling most stream valleys as well as sand with subordinate gravel and clay in the lowland terraces fringing the larger water bodies of Southern Maryland. All of these sediments are Pleistocene or younger in age.
The principal mineral resources of the area are constructional sand, gravel, and clay. Sand and gravel are abundant and widespread in the Upland Deposits and to a lesser extent in the lowland terraces bordering the Potomac and Patuxent Rivers. Thicknesses ranging from 15 to 25 feet of predominantly quartzose sandy gravel with little or no overburden are typical of the Upland Deposits over a large portion of the Southern Maryland area. Medium to coarse-grained qumtz sand, and to a lesser extent small gravel, is locally distributed in the Lowland Deposits, but the fact that much of this material is near or below the water table diminishes its economic potential to some degree. Production of aggregate, currently the major mineral industry in Southern Maryland, is likely to remain so in the foreseeable future. Some clays in the St. Marys Fonnation have t ested favorably for lightweight aggregate, and a ceramic clay suitable for face brick and structural tile is present in the Marlboro Clay. Deposits of diatomite, glauconite, and phosphorite also occur in the survey area but are not of economic value under present market conditions.