Geologic map of the Cowlitz Falls Quadrangle, Lewis and Skamania counties, Washington.

Abstract

The Cowlitz Falls Quadrangle, located about 25 km northeast of Mount St. Helens, is underlain by late Oligocene to early Miocene volcanic and volcaniclastic rocks locally veneered by Pleistocene glacial deposits and tephra derived from Mount St. Helens. The Tertiary rocks are chiefly basalt, andesite, and dacite flows and associated fragmental rocks deposited in a subaerial environment. They have been gently tilted to the east and intruded by several granitic bodies including the 21-Ma Spirit Lake Pluton. Extrusive and intrusive rocks are locally hydrothermally altered to clay-carbonate and advanced argillic assemblages. Thick glacial outwash deposits are present in the Cowlitz and Cispus valleys in the northern part of the quadrangle.

29 samples from this reference