If a low-angle thrust cut through the sial into the dunite of the sima the region affected is uplifted by thickening of the crust. This concentration of mass puts the crust locally out of isostatic balance. The measure of uplift may be the same throughout a section transverse to the strike, but the excess load will vary with the density of the rocks intersected on the dip. If recovery of equilibrium be slow and the uplifted surface be subjected to erosion, there will be notable hypsometric contrasts in the profile of the region, when it again attains isostatic balance. The surface in that part of the section where the thickening is wholly in the dunite will be much lower than in the part where the thickening is wholly in the sial. It is suggested that in this principle may be found the explanation of certain large structural valleys like that of the Great Valley of California.