The structural basement in the Rocky Mountains foreland deforms by faulting and rigid-body rotations. The faults at the interface between the sedimentary and crystalline rocks can be anything from low-angle reverse to normal faults. Despite the range of geometries, the total assemblage of faults and rotations is best explained by a movement system that is dominated by vertical motions along faults, many of which are curved in cross section. The first causes deep within the crust are not sufficiently well documented in the geophysical record to justify a firm interpretation. However, there are certain conditions recorded in the geologic history...