MARYLAND GEOLOGICAL SURVEY 197

Plain, with the possible exception of the Cretaceous. The deposits of
Neocene age have been divided into two formations, viz., the Chesa-
peake and Lafayette formations.

THE CHESAPEAKE FORMATION. —The Chesapeake formation, so
called from its extensive development upon the shores of the Chesa-
peake Bay, extends as a broad belt across the state and occurs like-
wise in the areas both to the north and the south of Maryland as an
important member of the Coastal Plain series. The strata overlie
unconformably those of the Pamunkey formation and gradually over-
lap the latter toward the north, and just beyond the Delaware border
rest directly upon the upper Cretaceous beds.

The deposits of the Chesapeake formation consist of sands, clays,
marls and diatomaceous beds, the latter composed chiefly of the tests
of the microscopic plant forms called diatoms, and mainly confined to
the lower portion of the formation. The diatomaceous beds afford
fine sections at Pope's creek on the Potomac, at the mouth of Lyon's
creek, a tributary of the Patuxent, and at Herring Bay on the west
shore of the Chesapeake. At these points the light colored bluffs are
very striking objects in the landscape. The nearly pure diatomaceous
earth reaches a thickness of about 30 feet, although the remains of
diatoms are found scattered in greater or less amounts throughout the
overlying strata. This diatomaceous earth can be traced from the
eastern shore of Maryland entirely across the state and thence south-
ward into Virginia. From its wide occurrence in the vicinity of Rich-
mond it is sometimes known as " Richmond earth. " It was long
referred to in the literature of the subject as " Bermuda earth, " from
its supposed occurrence on the Island of Bermuda, but the specimen
upon which the reference was based was ultimately shown to have
come from " Bermuda Hundred " on the James river. The diatoma-
ceous earth is frequently described under the names of " Infusorial
earth, " " Tripoli " and " Silica. " The higher portions of the Chesa-
peake formation are comprised of sands and clays of various colors
and frequently carry vast numbers of molluscan shells. Extensive
beds of shell marl underlie much of the Miocene country. These
deposits become at times cemented into hard limestone ledges. At