Geologic Map of the East Half of the Bellevue South 7.5’ x 15’ Quadrangle, Issaquah Area, King County, Washington

Introduction

The Issaquah area includes several of the most outstanding geologic features of the eastern Puget Lowland region. Folds have warped thousands of meters of Tertiary sedimentary and volcanic rocks. Several hundred meters of both glacial and postglacial sediment have accumulated in a deep glacial trough, which is now partly occupied by Lake Sammamish but which was previously the conduit for massive volumes of meltwater during ice-sheet occupation and retreat. The eastern projection of an east-west-oriented crustal structure, which reflects Tertiary through Holocene fault displacement, extends across the eastern part of the map area.

In addition to these geologic features, some of the most rapid human alteration of the landscape in the entire Puget Lowland has occurred here. Since the 19th century, coal was extensively mined and, since the early 1980s, the region has been overtaken by urbanization. In places, this alteration has dramatically accelerated the rate of geomorphic processes. For example, the hillsides have been regraded as a result of mining and quarries throughout the southern one-third of the quadrangle; stream channels have recently incised above the eastern shores of Lake Sammamish; and sediments have deposited on the lakeshore and into the lake itself.

First posted September 27, 2012