The term mountain is somewhat loosely employed for any lofty eminence, and the distinction between mountains and hills, as ordinarily made, is principally a question of height. Some so-called mountain peaks and ridges are merely the portions of dissected plateaus left standing, such as Lookout Mountain and Missionary Ridge in Tennessee, and the Alleghany Front in Pennsylvania. Such mountains usually have flat tops (table mountains), are composed of strata which are nearly or quite horizontal, and owe their existence either to their being composed of more resistant rocks than the denuded parts of the plateau, or to their favourable situation withreference to the drainage lines. Another type of mountain is the volcanic, which is usually an isolated cone and may be built up to great heights; it is simply the accumulation of volcanic material which has been piled up around the vent. To the same general class, as due to igneous rocks, might be referred the laccolithic mountains, in which an intrusive magma has pushed up the overlying strata into a dome.
Such mountains may stand isolated (Fig. 220), or several may be grouped together (Henry Mountains of southern Utah), or they may form extensive parts of true ranges (Elk Mountains, Colorado). Block mountains, which are tilted and eroded fault-blocks, form a third class; these may be single or in groups, or lineally extended as a range. Typical mountain ranges and chains are mountains of folding, and differ materially from any of these classes, both in their structure and their mode of origin. Before proceeding to discuss the origin and history of mountains, it will be necessary to define the terms to be used.
A Mountain Range is made up of a series of more or less parallel ridges, all of which were formed within a single geosyncline (p. 330) or on its borders. The ridges are separated from one another by longitudinal valleys, and may be formed either by the successive folds or by denudation within the limits of the folds. In the latter case the outcropping harder strata make the ridges. A mountain range is always very long in proportion to its width, and its ridges have a persistent trend. These features distinguish a true range from the ridges cut out of a plateau by denudation. The Appalachian range, the Wasatch, the Coast Range, are examples of typical mountain ranges.
A Mountain System is made up of a number of parallel or consecutive ranges, formed in separate geosynclines, but of approximately similar dates of upheaval. The Appalachian system comprises the Appalachian range, running from New York to Georgia, the Acadian range in Nova Scotia and New Brunswick, and the Ouachita range in Arkansas. Each of these ranges was formed in a different geosynclinal, but at the same geological date, and they are consecutive, having a common direction.
A Mountain Chain comprises two or more systems in the same general region of elevation, but of different dates of origin. The Appalachian chain includes the Appalachian system, the Blue Ridge, the Highlands of New Jersey and the Hudson, a system of different date, and the Taconic system of western New England, which was not formed at the same time as either of the others.
A Cordillera consists of several mountain chains in the same part of the continent. Thus, the chains of the Rocky Mountains, Sierra Nevada, Coast Range, and their prolongations in Canada, together make up the Rocky Mountain or Western Cordillera.
From these definitions it will appear that the mountain range has a unity of structure and origin which fits it especially for study. If the history of the ranges be understood, the systems and chains will offer little additional difficulty.
A mountain range (disregarding, for the present, certain exceptional cases) consists of a very thick mass of strata, which are much thicker in the mountains than the same strata in the adjoining plains. In the Appalachian range, for example, the stratified rocks are more than 25,000 feet thick, but on tracing the same series of beds westward into the Mississippi Valley, they are found to become very much thinner, hardly exceeding one-tenth of the thickness in the mountains. This immense thickness of the component strata is not peculiar to the Appalachians, but reappears in the typical mountain ranges everywhere; the Wasatch range has 31,000 feet of strata, the Coast Range 30,000 feet, the Alps 50,000 feet, etc. The thick series of strata which make up a mountain range are usually conformable throughout, though this conformity may in some cases be deceptive and due to the obliteration of unconformities by folding. Deposition usually appears to have been without conspicuous breaks, and there was little or no loss from denudation, though in some cases the region which subsequently was upheaved into the range had its oscillations of level, recorded now in unconformities.
This may be seen, for example, in the Ouachita range of Arkansas.
Another well-nigh universal fact concerning the structure of mountain ranges is the intense folding or plication of their strata, often accompanied by great thrusts. The degree of plication varies much in different ranges. The Uinta Mountains are formed by a single great and gently swelling arch of strata, faulted along its northern slope. So gentle is the curvature of the beds that in a single view they often seem to be quite horizontal. The Black Hills, South Dakota, form a great dome, with somewhat oval ground-plan. Much more commonly the strata are thrown into a series of parallel folds, which sometimes are open, upright, and symmetrical, as in some of the ridges of the Jura Mountains of Switzerland, in which the folds are so symmetrical and regular that a section across the parallel ridges looks like a diagram. This comparatively gentle folding is, however, not the rule, but rather an intense compression and plication. The Appalachians are thrown into closed, asymmetrical, and overturned folds, with frequent great thrusts. The Sierra Nevada is so intensely plicated that the thickness of its strata has not yet been estimated.
The Alps have undergone such enormous compression that many of the ridges are in the form of fan folds (i.e. the anticlines are broader at the crest than at the base), while others have been pushed over to an inverted position. The combination of this violent contortion with faults and thrusts often results in an indescribable confusion and chaos of forms, which it is exceedingly difficult to comprehend.
In folded mountain ranges three zones may be distinguished: (i) A rigid, unyielding mass which is not folded, (2) the zone of folding, (3) the zone of diminishing action, where the folding gradually dies away or ends in a fault. Many, perhaps most, ranges are bounded by faults on one or more sides, as is true of the Sierra Nevada, Wasatch and Uinta Mountains, the Alps, etc. The side of the range toward which the overturned folds incline is called the foreland, and may be either the unfolded mass or the zone of diminishing action; the former arrangement occurs in the Alps, the latter in the Appalachians.
The two main characteristic features of mountain ranges are, then, the immense thickness of the strata of which they are made, and the compression and folding or thrusting which they have undergone. Certain minor structures which accompany these more striking features should, however, not be overlooked. In the first place, the folded strata of mountain ranges are very generally cleaved, or fissile, or both, the planes of cleavage or fissility running parallel with the axes of the folds. (2) The major folds are themselves composed of successive series of minor folds in descending order of magnitude, the smallest of them being visible only with the microscope. (3) Dynamic metamorphism is an almost universal feature of mountain ranges, the transformation of the rocks being in proportion to the intensity of the plication. The microscope gives eloquent testimony to the enormous forces which have been at work, by showing how the minerals have been mashed and flattened, rendered plastic and flowing like wax in a hydraulic press. (4) Masses of igneous rocks are very often though not always, associated with mountain ranges, and many such ranges have a core of igneous rock, often granite, with strata flanking it on both sides.