In geology, the term compression refers to a set of stresses directed toward the center of a rock mass.
When the maximum compressive stress is in a horizontal orientation, thrust faulting can occur, resulting in the shortening and thickening of that portion of the crust.
When the maximum compressive stress is vertical, a section of rock will often fail in normal faults, horizontally extending and vertically thinning a given layer of rock.
[1] Compressive stresses can result in a number of different features at varying scales, most notably including Folds, and Thrust faults.
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