Fold-Thrusts are intimately connected with folds and occur only among folded sedimentary rocks; they may arise by plication and inversion, usually between an overturned anticline and the adjoining synclines. Thrusts of this character are very widespread and common in regions of strongly folded and plicated strata and represent the breaking and dislocation of rocks in the process of folding. The central and southern Appalachians, Arkansas, and Oklahoma, and the northern Rocky Mountains are the regions of the United States where great thrusts of this kind are most frequent. In the latter mountains, on both sides of the international boundary line, great fold-thrusts have carried masses of strata at least eight miles to the eastward. In fold-thrusts the plane of dislocation is somewhat steeper than in scission-thrusts and sometimes approximates the steepness of typical reversed faults. (See Fig. 49 p. 129).

Steep fold thrust, Big Horn mountains, Wyoming.

Fig. 184. - Steep fold-thrust, Big Horn mountains, Wyoming. (U. S. G. S.) Strata of hanging wall, on left of thrust-plane, show drag.