William H. Hobbs. Requisite Conditions for the Formation of Ice Ramparts, Jour. Geol., vol. 19, 1911, pp. 157-160.
CHAPTER XXXI
THE ORIGIN AND THE FORMS OF MOUNTAINS
A mountain defined.—As ordinarily understood, mountains are elevations upon the earth’s surface which rise above the general level of the country. Their summits need not be at great heights above the sea, but it is essential that they project above the average level of the surrounding country by at least a quarter of a mile. Lower elevations are described as hills. On the other hand, the elevation of a plateau like the “High Plains” of the western United States may be as much as a mile, but the vast expanse of nearly level surface precludes the use of the term “mountain.” The word is thus applied to a feature of the earth and not merely to an elevated tract.
In a collective sense, though more often in the plural form, the term is properly applied to groups of similar features which have a common origin in local uplift of the land. The origin of mountains used in this sense of mountain complexes is thus connected with some essentially local uplift of the earth’s surface. This may take place by the processes of folding and superincumbent fault displacement, by volcanic extravasations or ejections, or by a deeper seated and essentially hydrostatic elevation of rock beds over molten rock material.
The existing forms of mountains, as we are to see, are largely shaped by the erosional processes which are set in operation by the uplift itself, though often completed long subsequent to it.
The festoons of mountain arcs.—From our earliest studies of school geographies, we have become familiar with the arrangement of the more important mountains in long chains or systems. Comparatively few persons have given any further attention to the arrangement of the chains, though over large areas of the earth’s surface the distribution of mountain ranges is deeply significant. The map of Asia in particular presents a series of great sweeping arcs or crescents which are grouped as though hung upon the map in festoons with knots or vertexes to separate neighboring groups ([Fig. 474], [p. 438], and [Fig. 472]).
