Marlboro Clay

Geology

The Marlboro Clay, named by Darton (1948) for exposures near the Prince Georges County seat of Upper Marlboro, is a continuous stratum throughout nearly the whole of Southern Maryland. The Marlboro is poorly exposed, mostly because it is thin and readily covered by slumping of the overlying sediments. The outcrop belt is a narrow sinuous line which enters the area near Palmers Corner in Prince Georges County and trends southwest for some 20 miles to the vicinity of Rison in Charles County. In the broad valleys of Piscataway and Mattawoman Creeks, the clay is effectively buried beneath Holocene alluvium. Similarly, it disappears southwest of Rison beneath a thick mantle of Pleistocene Potomac River terrace deposits. Scattered patches of typically pale-red Marlboro clay can be seen in the much-overgrown cuts along Md. 210 just south of Hunters Mill Branch and just north of Piscataway Creek, both in Prince Georges County. An excellent exposure, perhaps the finest in Southern Maryland, is in a high cut bank a few yards southeast of Md. 224 just .6 mile southwest of Mason Springs in Charles County. The entire thickness of the Clay, in this case 14 feet, is exposed here as well as 10 feet of the underlying Aquia and several feet of the overlying Nanjemoy sand.

The Marlboro Clay is silvery-gray to pale-red plastic clay interbedded with much subordinate yellowish-gray to reddish silt. In most instances where the unit is relatively thick, the gray hues are characteristic of the uppermost and lowermost several feet of clay whereas the palered to reddish-brown colors are typically developed in the middle portion. The Marlboro Clay is thought to have been deposited in a shallow marine environment during the late Paleocene and early Eocene, approximately 55 million years ago.

Landslide susceptibility

In Maryland's Coastal Plain province the Marlboro Clay is one of the geologic formations highly susceptible to slope failure. Other formations highly susceptiple to slope failure include clays of the Potomac Group and the St. Mary's Formation. Both slumps and earthflows can occur associated with the Marlboro Clay. Slope failures are particularily numerous in south-western and east-central Prince Georges County (Pomeroy, 1988). As overlying permeable sediment becomes heavily saturated with infiltrating precipitation, the frictional resistance lowers along the contact with the low permeability Marlboro Clay producing a slide surface which could potentially lead to slope failure.

Darton, N.H., 1948, The Marlboro Clay: Econ. Geol., Vol. 43, p. 154-155.

Pomeroy, J.S., 1988, Map showing landslide susceptibility in Maryland: U.S. Geological Survey Misc. Field Studies Map 2048, 8 p., 1 map