The Amboy 7.5' quadrangle is situated in the foothills of the western Cascade Range approximately 50 km northeast of Portland, Oregon. Since late Eocene time, the Cascade Range has been the locus of an active volcanic arc associated with underthrusting of oceanic lithosphere beneath the North American continent along the Cascadia Subduction Zone. Volcanic and shallow-level intrusive rocks emplaced early in the history of the arc underlie the Amboy quadrangle, forming a dissected and glaciated terrain with elevations as high as 2050 ft (625 m). The quadrangle is transected by two troughs that roughly parallel the east- west structural grain. The northern trough is occupied by Lake Merwin, an artificial reservoir inundating the valley of the Lewis River. The Lewis drains a large area in the southern Washington Cascade Range, including the southern flank of Mount St. Helens, approximately 20 km upstream from the quadrangle, before joining the Columbia River about 25 km west of the quadrangle. The southern trough, which includes Chelatchie Prairie and the lower stretch of Cedar Creek, probably marks a former course of the Lewis River.