The distribution of Laramide strain in the Precambrian basement rocks of four small Rocky Mountain foreland folds was controlled by lithologies and orientations of preexisting foliation in the faulted forelimbs. Features of brittle deformation that developed in the basement were faults, sets of parallel, conjugate, or anastomosing fractures, zones of penetrative grain cracks and intergrain slip without grain-size reduction, and local zones of cataclasis or incipient mylonitization.

In the London Hills anticline of Montana, foliation in amphibolite and gneiss was nearly parallel to bedding in cover rocks prior to folding. The foliation in the forelimb was rotated and deformed by...