The crumpling of the earth's crust which folded the rocks of the Highlands and Southern Uplands probably upraised above the sea a series of longitudinal ridges having a general north-easterly direction.
There were, moreover, other and broader oscillations which raised or lowered extensive areas withbut much crumpling of the strata, and to these are due some of the most important breaks in the geological series.
Extensive and deep-seated crumpling was necessarily accompanied by vertical uplift throughout the zone affected, but once at least since their birth the mountains have been worn down to a lowland, and the mountains of to-day are the combined product of subsequent uplift of a different sort, and dissection by erosion.
This latest and also highest range was formed by tremendous thrusts from the Pacific side, crumpling and folding the ancient sedimentary rocks, which run from the Cambrian to the Cretaceous, and faulting them along overturned folds.
- The Alps form but a small portion of a great zone of crumpling which stretches, in a series of curves, from the Atlas Mountains to the Himalayas.
If a circular disk is wrought into a hemisphere and the attempt is made to hammer the edges round, crumpling must occur.
- The Himalaya have been formed by violent crumpling of the earth's crust along the southern margin of the great tableland of Central Asia.
Outside the arc of the mountain chain no sign of this crumpling is to be detected except in the Salt Range, and the Peninsula of India has been entirely free from folding of any importance since early Palaeozoic times, if not since the Archean period itself.
Upon this fault the mountains were pushed forward over the beds deposited at their base, crumpling and folding them in the process, and forming a sub-Himalayan ridge in front of the main chain.
In its smaller manifestation, this class of folding passes into "crumpling" or "puckering," where quite a large number of folds may be crowded into a single hand specimen.