1. Frequency folding such as results from inadequate sampling, producing alias (q.v.) problems. 2. Convolution (q.v.). 3. Bending of geological strata. Primary folding is a response to deep-seated forces with a strong horizontal component. Gravitational folding results from downward sliding and flow, which are secondary results of uplift and tilting. Local folding can be caused by compaction or by the upwelling of salt or igneous rocks. The upraised part of a fold forms an anticline, the downwarped portion a syncline. Folding terminology is shown in Figure F-17. Materials respond differently to the same stresses (and stress durations) and cleavage, flow, and faulting are usually associated with folding; see Figure F-17. Competent beds tend to retain their thickness in folding; they govern the folding wavelength, which is of the order of 25 times the thickness of the most competent member. Incompetent beds flow in response to folding stresses, but the distinction with competent beds is gradational. Two folding styles are illustrated in Figure F-17d, e. Folding is disharmonic where the folding of one bed is not geometrically related to the folding of nearby beds, incompetent beds intervening between them.