In burial of strata there is a possibility whereby they wrinkle, and wrinkle most in one direction, which piston pressure does not imitate. That is the heating by burial and expansion or lengthening of controlling layers. In a long basin like the Appalachians, the wrinkling under expansion across the greatest length is easiest, because the axis of stiffness is parallel to the long trough. Transitions off the coastal line from one sediment to the next--sand to mud, mud to lime--will be weaknesses to start bends when expansion pressure takes place under burial along the layers separately heated. These bends develop into wrinkles and the wrinkles, into propagated folds, with the axis parallel to the initial change of weaknesses. Expansion lengthwise on folds, once begun, may make long flat arches pitching in one direction. This heating by burial distributes the folding better and farther than pushing abutments, and makes initial bends. All bottom strata heat and expand in all directions. The direction of easiest yielding to a folding impulse is across the weak transition belts. After that the motion is taken up by linear folds and fractures in one direction. The models, after continuous or intermittent squeezing, were removed from the chest and sliced with a hot wire for sectioning and photographing. In one, brittle, broken series of folds in a hard layer, the model was taken apart on that layer and the surface photographed. The crest of the folds showed jointing or regular cracks. One set paralleled the fold axes as would be expected; the other set crossed the slopes diagonally and in curves. These last indicated the strains of a twisting nature on a single layer between a downfold and an upfold. What makes the end thrust, or piston push, in nature? According to the old idea, it was contraction of the inner earth by loss of heat. Willis wrote that the basin sank, isostasy or deep flow was at right angles to the length of the basin, and general contraction took effect by reason of the deep flow. The deep flow was toward the lighter continent, from which the sands were originally lost.