Folding of rocks has two main effects when denudation shapes the rocks into landforms. Firstly, the rocks are tilted and warped. Beds of resistant rock give persistent structurally-controlled slopes in the landscape. Cuestas may develop where rock resistance is unequal, with the resistant bed forming an inclined slope parallel to the dip of the rock (dip slope) and a strike-aligned scarp. Secondly, the flexing brings the bed of rock into tension or compression. Up-folds or anticlines cause stretching and fracturing of rocks at their core, allowing more rapid erosion and breaching of the anticline. Down-folds or synclines have closed fractures and tend to resistant to erosion, leading eventually to the base of the fold forming a hill, a perched syncline.