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

author

Booth, D.B., Walsh, T.J., Goetz Troost, K., and Shimel, S.A., 2012, Geologic map of the east half of the Bellevue South 7.5’ x 15’ quadrangle, Issaquah area, King County, Washington: U.S. Geological Survey Scientific Investigations Map 3211, scale 1:24,000. (Available at https://pubs.usgs.gov/sim/3211/.)

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.

Source Lake Falls (waterfallsnorthwest.com) The South Fork of the Snoqualmie River officially starts at Source Lake in the basin just north of the Alpental Ski Area at Snoqualmie Pass. Though diminutive in size its outflow is fed by several rocky alpine basins higher up on the ridgeline below Bryant and Chair Peaks which punctuate the skyline in the Alpental area. As the fledgling river meanders out of the lake and through sub-alpine huckleberry meadows, it encounters a very nondescript headwall and inconspicuously plunges over a three-stage waterfall which were it situated along the nearby Snow Lake Trail would be one of the marquis destinations for hikers in the immediate Snoqualmie Pass area.