158 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

In the Piedmont Plateau the twofold character of the district is
very marked geologically. On the east side of the central watershed
(Parr's Ridge) we have a sequence of highly crystalline rocks, in large
part igneous in their origin, which represent the remains of a vast
Archean continent, whose detritus furnished much of the material of
which the Paleozoic sediments were made. On the western side of
the median ridge the rocks are as yet only partly crystallized and
represent the greatly folded and metamorphosed beds of early Paleo-
zoic time. Along the western edge of this plateau, beyond the
Monocacy river, is the Frederick valley composed of the blue Paleo-
zoic limestone, in part overlain by the red sandstone and shale of
Mesozoic age.

The threefold division of the Appalachian Region corresponds
approximately to the threefold division in the sequence of the Paleo-
zoic strata. The Blue Ridge and Great Valley are made of Cambrian
and lower Silurian beds, in places so developed or eroded as to expose
the Archean floor on which they rest; the Appalachian Mountains
proper are made up of sharply folded upper Silurian and Devonian
strata, each easily recognized by its characteristic life forms; while the
Alleghany Mountains are mainly composed of more gently folded later
Devonian and Carboniferous deposits, carrying the valuable coal
seams of the Cumberland basin.

Such, in brief, is the distribution of the geologic formations in
Maryland and their connection with the easily recognized types of
surface configuration occurring within the state. The sequence is of
remarkable completeness and of great interest on account of the many
types of topography and soils which the various formations produce.
In the succeeding pages the geological history of each of the three
provinces—plateau, mountains and coast plain—will be traced out in
more detail. A somewhat different sequence will be followed than
in the preceding physiographic sketch, the Piedmont Plateau being
considered first, as it is the oldest, and then in order the Appalachian
Region, which is next in age, and finally the Coastal Plain, which is
the youngest portion of the state. Constant reference to the geo-
logical map will be found of service in following the descriptions
which will be given.