The geography of Pennsylvania by Redway Jacques W

Author:Redway, Jacques W[ardlaw], 1849- [from old catalog]
Language: eng
Format: epub, pdf
Publisher: Syracuse, N.Y., C. W. Bardeen
Published: 1891-03-25T05:00:00+00:00

but ofteiier many sucli folds are ranged side by side. Usually the various folds are nearly parallel, forming a belt that extends many miles in length.

Sometimes, however, instead of a series of long, regular folds, there is an irregular cluster or group of highlands in which short ridges have been thrust together almost as one would crumple paper with the hand. The Alps, the Adirondack and the White Mountains are good examples of this kind of formation.

The single fold of the earth's crust is commonly called a mountain-ra7ige, and the various ranges which compose a great mass form a mountain-system. The top of a ridge or range is its crest. A part of the crest which rises above the average height is called Q>peah. Isolated mountain peaks rising from a level plain are of very rare occurrence, and are always volcanic in origin.

It is now generally thought that mountains were formed by the shrinking of the earth's crust over a cooling interior. As the interior of the earth parted with its heat, it contracted or became smaller in bulk. As a result the outer parts, in shrinking over the contracting mass became wrinkled and crimpled, just in fact as the skin of an apple shrivels and folds when the pulp contracts, because of its loss of bulk.

It has required many ages for the formation of mountain folds, and it is more than likely that the crumpling process is still going on in even the oldest mountain-systems. "We can best understand how slowly the process of mountain-formation has gone on when we learn how the Delaware, Lehigh, Schuylkill, Susquehanna and Swatara Rivers have cut their channels through the Blue Mountains.

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