Abstract

In northeastern Tidewater Virginia and in southern Maryland the gray unweathered marine marls, clays, and sands of the Chesapeake group (Miocene) exposed in the high bluffs on Rappahannock and Potomac rivers and on Chesapeake Bay are overlain by 60-80 feet or more of sand and gravel. Previous investigators have treated this sand and gravel as one unit and referred it either to the so-called Lafayette formation (Pliocene?) or to the Sunderland formation (Pleistocene). We divide the sand and gravel unit into two formations.

The lower formation consists mainly of 40-60 feet of strikingly soft, loose, medium to coarse sand with interbedded lenses of gravel and subordinate light-gray clay partings. Glauconite is present in some beds. The basal bed is a fine gravel that rests unconformably on the unweathered Miocene strata, transgressing northwestward across both the St. Marys and the Choptank formations. Locally the materials are indurated to ferruginous sandstone and conglomerate, and one such mass at Nomini Cliffs (Horsehead Cliffs), Virginia, yielded an impression of Halymenites major Lesquereux. This fossil, representing a supposed marine organism, and the glauconite, the clay laminae, and the soft, loose texture of the sand, indicate the shallow marine origin of the formation.

Unconformably above the marine sand in the higher bluffs is 10-25 feet or more of medium to coarse, more or less ferruginous sand and lenses of gravel, with scattered pebbles and cobbles of quartz and occasional boulders of crystalline rock along the base (Pleistocene terrace deposits). These sands and gravels are locally indurated with iron oxide cement. In the higher bluffs (120-160 feet high) these deposits are interpreted to be the Sunderland formation; in the lower bluff (94 feet high) south of Jones Point on Rappahannock River, Virginia, they may belong to the Wicomico formation.

Fossils have been found in the Yorktown formation within a few miles of the lower sand and gravel formation, at altitudes that would permit the latter to fall within the stratigraphic limits of the Yorktown. The lower formation is therefore considered a northward extension of the Yorktown formation (Miocene).

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