Among the various processes by which mountain ranges have been formed, the folding and accompanying general uplift of strata are the most important. In fact, in most of the great mountain ranges of the world the folded structure is conspicuously developed, so much so that they may well be called "folded mountains." Very commonly, however, mountains of this type have also been subjected to more or less fracturing of the rocks (faulting), and not uncommonly they have also been subjected to igneous activity, including both intrusion and extrusion of molten material. It is among the folded mountains of greater or less degree of complexity that the "greatest exhibitions of geologic phenomena are seen and the lessons which geology as a sciences teaches may be learned. If one desires to know the history of a region, one turns naturally to its mountain ranges, for here may be found the upturned and dissected strata, a study of whose kinds, thickness, and fossils throws light upon past events, while their foldings and dislocations show the nature and results of those great dynamic agencies which, from time to time, have operated upon the outer portion of the earth, and given to it the broad distinctive features which characterize it to-day." (L. V. Pirsson.) Among the great mountains we may also see wonderful exhibitions of the results of weathering and erosion, especially the work of rivers and glaciers.
We can, perhaps, best convey to the reader some of the main facts and principles regarding folded mountains by considering certain observations which may be readily made in a short trip across a folded range of not too complex kind--for example, across the Appalachian range along the line of the Baltimore and Ohio Railroad, west of Washington, or the Pennsylvania Railroad, west of Philadelphia. It would be most evident that the mountains consist of strata, that is sedimentary rocks, such as sandstone, shale and limestone, which were deposited under water. A few measurements would reveal the fact that thousands of feet in thickness of strata are represented. Careful measurements by geologists have, in fact, shown that the strata were originally piled up layer upon layer to a thickness of 25,000 to 30,000 feet. The fact that they are strata of such great thickness proves that sediments must there have accumulated under water for some millions of years at least. Closer examination of a few good exposures (i.e., outcrops) would further reveal the presence of fossil shells and impressions of marine organisms, thus definitely leading us to conclude that the strata were accumulated under sea water, which, of course, means that the present site of the mountain range was once sea floor.