Like all processes which take place deep within the interior of the earth, the causes of crustal deformations are very obscure and there is much difference of opinion concerning them. The view which is held as to the physical state of the earth's interior will necessarily condition the explanation of folding and faulting, which is but one special aspect of the general problem. Any complete theory must of course contain a satisfactory solution of all the problems involved, but such a theory has still to be propounded and for the present we must be content with tentative hypotheses.

The first step in the inquiry is to determine the direction in which the folding force acted. At first sight, it might seem natural to suppose that the direction of the force was vertically upward, acting with maximum intensity beneath the anticlines and with minimum intensity beneath the synclines. But such an explanation could apply only to open, symmetrical, and simple folds, and even in these cases is not satisfactory. Folded strata must either occupy less space transversely than they did before folding, or else they must have been stretched and made much thinner, but a comparison of continuous beds, in the flexed and horizontal parts of their course, shows no such thinning. Again, such an explanation is obviously insufficient to account for closed, inclined, and inverted folds, for contortions and plications, and for flexures of different orders, one within another.

If the folding force did not act vertically, it must have acted horizontally, and this is the explanation now almost universally accepted. A horizontally acting force would compress and crumple up the beds, producing different types of flexure in accordance with varying circumstances. Furthermore, the microscopic study of intensely folded rocks shows that they have actually been compressed and mashed, and the minutest plications are visible only under the microscope.

Assuming, then, that the folding force was one of compression and acted horizontally, we have next to consider the circumstances which modify the result, producing now one form of flexure or fractare, now another. Such modifying circumstances are the depth to which a given stratum is buried, its thickness and rigidity, the character of the beds which are above and below it, and the intensity and rapidity with which the flexing force is applied. When in a mountain region one sees the manner in which vast masses of rigid strata are folded and crumpled like so many sheets of paper, one perceives the enormous power which is involved in these operations and the gradual, steady way in which that power must have been exerted. When strata are buried under a sufficient depth of overlying rock to crush them, they become virtually plastic and yield to the compressing force by bending. The movement would seem not to be a true molecular flow, but rather a gliding of the mineral particles one upon another. At such relatively great depths cavities cannot exist, and if the compressed rock should be broken by the compression, the particles are again welded together into a firm mass.

We may accordingly distinguish a shell of flowage, in which the rocks all yield plastically, a more superficial shell of fracture, in which all but the softest rocks break on compression, and between the two a shell of fracture and flow age, in which some rocks break and others bend, according to their rigidity. The depth of the zone of flowage is estimated at 20,000 to 30,000 feet below the surface.

Fig. 187. - Folded and fractured iron ore and jaspilite, Lake Superior region.

About 1/2 natural size.

Strata which have not been buried to a sufficient depth to make them plastic, will yield to compression by breaking, though whether a given bed is faulted or flexed, will often depend upon whether the folding force is applied slowly or with comparative rapidity. A force long acting in a slow and steady fashion will produce folds, when the same force applied more suddenly would shatter the beds. Near the surface, under light loads, rigid rocks will always break rather than bend, when compressed. Different stratified rocks differ much in their rigidity, and hence a load which is sufficient to cause one bed to bend and flow, when laterally compressed, will leave another unaffected, or cause it to break, if the compressing force overcomes its strength. In Bald Mountain, New York, the stiff limestones are left unchanged by a pressure which has crumpled and contorted the soft shales.

Fig. 188. - Plicated beds on unfolded ones; Mineral Ridge, Nevada. (U. S. G. S).

A certain amount of gentle folding may take place immediately at the surface and has actually been observed in process of formation, even in rigid rocks. In Wisconsin the limestone bed of the Fox River suddenly arched upward into a low anticline, crushing and bending the steel columns of a mill which had been built at that spot. The bed of the Chicago drainage canal, also in limestone, curved upward in similar fashion, when the excavation had removed the overlying load. Very many other surface folds are demonstrably of very recent origin. Folds of this character are due to a gentle and gradual compression, to which the strata yield by a readjustment of the joint-blocks of which they are made up.

Fig. 189. - Plicated limestone, with sheet of igneous rock, near Rockland, Maine. (U. S. G. S.) The limestone has flowed under compression, and the igneous rock has fractured.