These Permian ground moraines are not the first traces of the work of glaciers met with in the geological record.  Similar deposits prove glaciation in Norway succeeding the pre-Cambrian stage of elevation, and Cambrian glacial drift has recently been found in China.

The Appalachian deformation.  We have seen that during Paleozoic times a long, narrow trough of the sea lay off the western coast of the ancient land of Appalachia, where now are the Appalachian Mountains.  During the long ages of this era the trough gradually subsided, although with many stillstands and with occasional slight oscillations upward.  Meanwhile the land lying to the east was gradually uplifted at varying rates and with long pauses.  The waste of the rising land was constantly transferred to the sinking marginal sea bottom, and on the whole the trough was filled with sediments as rapidly as it subsided.  The sea was thus kept shallow, and at times, especially toward the close of the era, much of the area was upbuilt or raised to low, marshy, coastal plains.  When the Carboniferous was ended the waste which had been removed from the land and laid along its margin in the successive formations of the Paleozoic had reached a thickness of between thirty and forty thousand feet.

Both by sedimentation and by subsidence the trough had now become a belt of weakness in the crust of the earth.  Here the crust was 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 mashed them.