180 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

as the Shenandoah (upper part), Martinsburg, Juniata, Tuscarora,
Rockwood and Lewistown formations.

THE SHENANDOAH FORMATION (upper part). —The Shenandoah
formation, which has already been described in the previous chapter
as in part of Silurian age, contains an abundant fauna of fossil
brachiopods, gasteropods, corals and crinoids in its upper beds. These
forms are in the main identical with those found in the Trenton lime-
stone of New York. As already stated, the line of separation be-
tween the Cambrian and Silurian portions of this formation cannot be
definitely determined, since no physical break occurs. The upper
portion of the formation does not differ materially from the lower,
so that the description which has been given will serve for present
needs.

THE MARTINSBURG FORMATION (Hudson river shales). —The
Martinsburg formation, so called from its typical development in the
vicinity of Martinsburg, West Virginia, occurs in several areas along
the western border of the Hagerstown valley and in the region imme-
diately adjacent to it upon the west. This formation consists of black
and gray calcareous and argillaceous shales, which are fine-grained
and show but slight variations within the limits of the state of Mary-
land. The shales bear from five to twenty per cent of carbonate of
lime. The deposits were formed in shallow seas which abounded in
graptolites, corals, brachiopods and trilobites which have left abund-
ant fossil remains. The fauna is essentially that of the Hudson river
shales of New York. The thickness of the formation varies from
700 to 1000 feet.

There has been but slight alteration in the shale, which is usually
not sufficient to obscure the bedding which, however, was never
sharply marked. The rocks of this formation have suffered consid-
erable decay as the result of the solution of the carbonate of lime con-
tained in them.

THE JUNIATA FORMATION (red Medina sandstone). —The Juniata
formation, so called from its typical occurrence upon the Juniata
river in Pennsylvania, is limited to the western portion of the central
division of the Appalachian Region in western Allegany county. It