now made of layers to the depth of six or seven miles. In
comparison with the massive crystalline rocks of Appalachia on the
east, the layered rock of the trough was weak to resist lateral
pressure, as a ream of sheets of paper is weak when compared with
a solid board of the same thickness. It was weaker also than the
region to the west, since there the sediments were much thinner.
Besides, by the long-continued depression the strata of the trough
had been bent from the flat-lying attitude in which they were laid
to one in which they were less able to resist a horizontal thrust.

There now occurred one of the critical stages in the history of
the planet, when the crust crumples under its own weight and
shrinks down upon a nucleus which is diminishing in volume and no
longer able to support it. Under slow but resistless pressure the
strata of the Appalachian trough were thrust against the rigid
land, and slowly, steadily bent into long folds whose axes ran
northeast-southwest parallel to the ancient coast line. It was on
the eastern side next the buttress of the land that the
deformation was the greatest, and the folds most steep and close.
In central Pennsylvania and West Virginia the folds were for the
most part open. South of these states the folds were more closely
appressed, the strata were much broken, and the great thrust
faults were formed which have been described already. In eastern
Pennsylvania seams of bituminous coal were altered to anthracite,
while outside the region of strong deformation, as in western
Pennyslvania, they remained unchanged. An important factor in the
deformation was the massive limestones of the Cambrian-Ordovician.
Because of these thick, resistant beds the rocks were bent into
wide folds and sheared in places with great thrust faults. Had the
strata been weak shales, an equal pressure would have crushed and