The most severe deformation occurs where continent-continent collision has taken place. When a group of mountains which are so aligned are clearly related with respect to composition and origin, they constitute a mountain range or system . The latter term generally applies to a grouping of several ranges which appear to be similar in form, structure, and alignment. The Cascade Range, Sierra Nevada, Rocky Mountains, and Canadian Rockies, among many other ranges, are all members of the American Cordillera system. It has been recognized by geologists for over a century that this basic association is important to the formation of mountains and the evolution and development of continents.
Mountain ranges are the consequence of the collision of lithospheric plates.
This type of range is characterized by a series of broad alternating synclinal and anticlinal folds which involve deformation of only the cover rocks. This type of mountain range is characteristic of deformation in the foreland where the cover has slid like a rug over a polished floor. The valleys have developed by erosion within the least resistant strata, such as limestones and shale. In contrast, the ridges are formed by very resistant rocks. The Valley and Ridge of central Pennsylvania is an example of this type of range.