MARYLAND GEOLOGICAL SURVEY 199

Pleistocene strata, they have been considered to represent the Plio-
cene, and thus to belong to the Neocene. Where they are exposed as
a superficial covering at the higher levels the strata exhibit much more
extensive changes than the Pleistocene deposits, so that it is probable
that much time must have elapsed after the former were laid down
before the deposition of the latter. The chronological determination
of the Lafayette formation is based, therefore, upon geological rather
than paleontological grounds.

THE PLEISTOCENE PERIOD.

Superficially overlying the other formations of the Coastal Plain
are deposits of Pleistocene age, which with marked variation in thick-
ness, composition and structure extend from the glacial accumulations
of northern New Jersey through the southern Atlantic and Gulf states
to the Mexican border. The deposits of the Pleistocene in Maryland
have been grouped in a single division, known as the Columbia
formation.

THE COLUMBIA FORMATION. —The Columbia formation, so called
from its characteristic development in the District of Columbia,
widely covers the other deposits of the Coastal Plain throughout the
eastern and southern counties of the state, and at the border of that
region extends far along the various stream channels which enter from
the Piedmont Plateau. The higher area of the central and western
portions of the Coastal Plain were apparently never reached by the
Columbia seas.

The deposits of the Columbia formation consist of gravels, sands
and clays, which differ from the preceding deposits of the Lafayette
formation in containing much less decomposed materials and in having
suffered far less denudation since they were deposited. The deposits
are generally coarsely stratified, with frequent occurrence of cross-
bedding.

The Columbia period is divisible into an earlier and a later sub-
division, represented in deposits occupying high and low levels
respectively. The earlier deposits are more nearly similar to those
of the Lafayette in color and in the greater frequence of decomposed