Another mode of origin of mountains is by the rise of molten material to the surface, especially where a chain of volcanoes is located. Thus the Cascade Mountains from northern California through Oregon and Washington, including Mounts Lassen, Shasta, Pitt, Baker, St. Helens, and Rainier, are very largely the result of volcanic action. The long chain of Aleutian Islands of Alaska, referred to in our study of volcanoes, is an excellent example of a great mountain range now being built up out of the sea by volcanic action. More locally molten rocks under pressure may not reach the surface but instead simply bulge or dome the strata over them, as in the case of the group known as the Henry Mountains of Utah, and also in other parts of the West.
In still other cases mountains of considerable area and altitude have resulted from erosion of uplifted regions where the uplift has been practically unaccompanied by either folding, faulting, or igneous activity. Any low-lying area, regardless of the character of its rocks, structure, or previous history, may be notably upraised and simply subjected to erosion. An excellent illustration is afforded by the Catskill Mountains of New York, where numerous deep valleys and narrow ridges have been carved out of upraised nearly horizontal strata. The so-called “Bad Lands” region of parts of South Dakota and Wyoming is also essentially of this type, where deep, narrow valleys and sharp ridges have been etched out of high, relatively soft, nearly horizontal strata, resulting in an almost impassable maze of mountains. In the high, recently upraised Colorado Plateau of parts of Arizona, New Mexico, Colorado, and Utah, nearly horizontal strata are being etched out, the result being numerous buttes, mesas (flat-topped hills and mountains) and deep canyons, including the Grand Canyon with its maze of peaks and pinnacles, many of them rising like mountains out of the canyon depths.
Mountains of the pure types just described are not the prevailing ones of the earth. Most mountains and their structures, as we see them to-day, are products of two or more of the processes of folding, faulting, igneous action, and erosion. A few well-known examples will suffice to make this matter clearer. Thus, the Appalachian Mountains originally developed by severe folding of thick strata. After considerable erosion, numerous small and large thrust faults developed, some of the dislocations amounting to miles. Then the whole range was cut down nearly to sea level by erosion, after which the district was upraised (without folding) mostly from 2,000 to 4,000 feet, and the present long, narrow mountain ridges and valleys have been carved out by stream erosion. Thus folding, faulting, and erosion all enter into the height and structure of the Appalachians.
A lofty mountain range still more complex in its history is the Sierra Nevada of California. First, thick strata were highly folded, upraised, and intruded by great masses of molten granite. Erosion then proceeded to cut the range down to hills, after which a great fracture (fault) developed along the eastern side and the Sierra Nevada earth block was notably tilted with steep eastern front and long western slope. Erosion has considerably modified the eastern fault face, and the deep canyons like Yosemite, King’s River and American River, have been carved out of the western slope of the great tilted fault block. Geologically recently the central to northern portion of the range has been affected by volcanic action, streams of lava in some cases having flowed down the valleys.
A STUDY OF LAKES
L
LAKES are ephemeral features on the face of the earth. Compared to the tens of millions of years of known earth history, lakes, even large ones, are very short lived. They may, in truth, be regarded as merely results of the temporary obstructions to drainage. Lake basins are known to originate in many ways, and there are various means by which they are destroyed. Not attempting an exhaustive, scientific treatment of the subject, our present purpose may be well served by describing and explaining some of the better known and more remarkable lakes of the world.