hard and soft layers which have been crumpled up much as one
might wrinkle a rug with one's foot. The pressure involved in the
process changed and hardened the rocks so much that the coal
which they contain was converted into anthracite, the finest coal
in all the world and the only example of its kind. Even the
famous Welsh coal has not been so thoroughly hardened. During a
long period of erosion the tops of the folded layers were worn
off to a depth of thousands of feet and the whole country was
converted into an almost level plain. Then in the late geological
period known as the early Tertiary the land was lifted up again,
and once more erosion went on. The soft rocks were thus etched
away until broad valleys were formed. The hard layers were left
as a bewildering succession of ridges with flat tops. A single
ridge may double back and forth so often that the region well
deserves the old Indian name of the "Endless Mountains."
Southwestward the valley grows narrower, and the ridges which
break its surface become straighter. Everywhere they are
flat-topped, steep-sided, and narrow, while between them lie
parts of the main valley floor, flat and fertile. Here in the
south, even more clearly than in the north, the valley is
bordered on the east by the sharply upstanding range of the
crystalline Appalachians, while on the west with equal regularity
it comes to an end in an escarpment which rises to the Alleghany
plateau.

This plateau, the third great band of the Appalachians, begins on
the south side of the Mohawk Valley. To the north its place is
taken by the Adirondacks, which are an outlier of the great
Laurentian area of Canada. The fact that the outlier and the
plateau are separated by the low strip of the Mohawk Valley makes