|FOLD, ANGULAR AND CUSPATE|
If the ductility contrast between the layer and the matrix is high, the matrix does not afford resistance to buckling and the process takes place rapidly and the initial thickness of the layer does not change and ptygmatic structures (the photograph being from the village Bhadsi in Delhi orogenic belt ) are formed. The initial wavelength selected for buckling at the onset of the process changes very little.
Folds in which the cross sectional shape of adjacent antiform-synform pairs alternates from a broad rounded hinge zone style (lobate) to a sharp crested hinge zone style (cuspate). Such folds are formed as a
result of contraction along an interface between more competent and less competent rocks, the lobate folds have cores of competent material, the cuspate folds have cores of less competent material. If not at an interface but in single layers, then matrix which is less competent occurs in the cores of antiforms as well as synforms. The fold at right shows that it has nearly the same photographic tone in case of layer and matrix (locality, on way to lighthouse at Rhoscolyn, Anglesey, N Wales).
If the layer undergoes shortening and buckles at a point of deflection, the form it takes depends upon the ductility contrast between the layer and the matrix. If the contrast is not well marked, the matrix affords resistance to buckling which in essence is the increase in limb dip, and the layer goes on thickening with no perceptible buckling until a critical stage or limb dips are reached which make the layer parallel strain meaningless. The folds formed have rounded outer and cusp like inner arcs, the initial span gets considerably changed and cuspate folds are formed. Note that the cuspate folds near Rhoscolyn have a composition very close to that of the matrix.