=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 (Figs. 169 and 180) 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 (Figs. 167 and 172) 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 (Fig. 173). [Illustration: Fig. 172. Overturned Fold, Vermont] In folds where the compression has been great the layers are often found thickened at the crest and thinned along the limbs (174). 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. [Illustration: Fig. 173. Fan Folds, the Alps] =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. [Illustration: Fig. 174. Folds with Layers thickened at the Crest and thinned along the Limbs] =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. [Illustration: Fig. 175. Relief Map of the Northern Appalachian Region From Bingham's _Geographic Influences in American History_]