Degrees of folding.  Folds vary in degree from broad, low swells, which can hardly be detected, to the most highly contorted and complicated structures.  In symmetric folds the dips of the rocks on each side the axis of the fold are equal.  In unsymmetrical folds one limb is steeper than the other, as in the anticline in Figure 167.  In overturned folds one limb is inclined beyond the perpendicular.  Fan folds have been so pinched that the original anticlines are left broader at the top than at the bottom.

In folds where the compression has been great the layers are often found thickened at the crest and thinned along the limbs.  Where strong rocks such as heavy limestones are folded together with weak rocks such as shales, the strong rocks are often bent into great simple folds, while the weak rocks are minutely crumpled.

Systems of folds.  As a rule, folds occur in systems.  Over the Appalachian mountain belt, for example, extending from northeastern Pennsylvania to northern Alabama and Georgia, the earth’s crust has been thrown into a series of parallel folds whose axes run from northeast to southwest (Fig. 175).  In Pennsylvania one may count a score or more of these earth waves,—­ some but from ten to twenty miles in length, and some extending as much as two hundred miles before they die away.  On the eastern part of this belt the folds are steeper and more numerous than on the western side.

Cause and conditions of folding.  The sections which we have studied suggest that rocks are folded by lateral pressure.  While a single, simple fold might be produced by a heave, a series of folds, including overturns, fan folds, and folds thickened on their crests at the expense of their limbs, could only be made in one way,—­by pressure from the side.  Experiment has reproduced all forms of folds by subjecting to lateral thrust layers of plastic material such as wax.

Vast as the force must have been which could fold the solid rocks of the crust as one may crumple the leaves of a magazine in the fingers, it is only under certain conditions that it could have produced the results which we see.  Rocks are brittle, and it is only when under a heavy load and by great pressure slowly applied, that they can thus be folded and bent instead of being crushed to pieces.  Under these conditions, experiments prove that not only metals such as steel, but also brittle rocks such as marble, can be deformed and molded and made to flow like plastic clay.

Zone of flow, zone of flow and fracture, and zone of fracture.  We may believe that at depths which must be reckoned in tens of thousands of feet the load of overlying rocks is so great that rocks of all kinds yield by folding to lateral pressure, and flow instead of breaking.  Indeed, at such profound depths and under such inconceivable weight no cavity can form, and any fractures would be healed at once by the welding of grain to grain.  At less depths there exists a zone where soft rocks fold and flow under stress, and hard rocks are fractured; while at and near the surface hard and soft rocks alike yield by fracture to strong pressure.