When rocks are subjected to stresses in the earth's crust they may bend or break forming geologic structures called folds and faults. A variety of geologic structures are possible depending on the rock types involved and the conditions present. Rocks made of brittle minerals such as quartz tend to fracture under stress yielding faults.  Rocks made of more ductile minerals such as clay minerals tend to bend under stress yielding folds. Rocks which are deeply buried (even brittle ones) are likely to fold under the great pressures at depth, whereas rocks nearer the surface (even ductile ones) are likely to fault under the reduced pressures near the surface. Time also plays a role in determining if a rock will fault or fold. Rocks which are subjected to rapid stresses are more likely to fault, whereas rocks subjected to stresses over long periods of time are more likely to fold.