§ 1. In this study only those folds as observed in the upper sedimentary layers of the Earth’s crust will be treated. Even with this restriction the subject is still so complicated, that we must narrow the scope to be able to treat it from a theoretical point of view. Hence we will confine ourselves to simple folds as synclines and anticlines, leaving alone those intricate structures generally known as Alpine folding. On the other hand faults connected with folding are such a common feature that any theory that does not take them into account will be valueless. Folding is the result of tangential forces acting at unknown depth. Only in very few instances we have some idea at what depth the force is active. In the Jura mountains for instance we know that the sedimentary layer must have been pushed over its granite basement rock, as it has glided over the granite on a very incompetent layer of Anhydrite. But such knowledge of the total depth of a system of folds is an exception.