Geologic map of the Snoqualmie Pass 30 x 60 minute quadrangle, Washington

The Snoqualmie Pass quadrangle lies at the north edge of a Tertiary volcanic and sedimentary cover, where the regional structural uplift to the north elevated the older rocks to erosional levels. Much of the quadrangle is underlain by folded Eocene volcanic rocks and fluvial deposts of an extensional event, and these rocks are overlain by Cascade arc volcanic rocks: mildly deformed Oligocene-Miocene rocks and undeformed younger volcanic rocks. Melanges of Paleozoic and Mesozoic rocks are exposed in structural highs in the northern part of the quadrangle. The quadrangle is traversed north to south by the Straight Creek Fault, and the probably partially coincident Darringon-Devils Mountain Fault. A rich Quaternary stratigraphy reveals events of the Frazer glaciation.