Before the time of the Laramide orogeny, an active orogenic and magmatic system was more or less continuous along the continental margin of western North America and had long dominated the Cordileran geologic framework; Laramide events reflected a major break in that continuity and were unusual in several respects. Contractional Laramide orogenesis affected a very wide zone, with deformation and foreland uplift extending nearly to the middle of the continent. Laramide magmatism, too, although discontinuous along strike of the orogenic region, extended locally far eastward. These events may have occurred in response to rapid westward drift of the North American Plate and extreme flattening of the Farallon subduction zone (Dickinson and Snyder, 1978).