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.
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.
