Preliminary Report on the Geology of Southern Snohomish County (PDF)

description

This report from 1973 describes the geology of southern Snohomish County.

excerpt

The map area includes eighty-five square miles in South Snohomish County southeast of Everett, bounded on the west by Interstate 5, on the south by Highway 405 and the Snohomish-King County boundary, on the east and north by the Snohomish River floodplain.

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pdf

https://www.dnr.wa.gov/Publications/ger_ofr73-1_geology_south_snohomish_co_pt1.pdf

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The Whidbey Formation is characteristically compact, bedded, bluish to gray-brown silt and clay. Thin sand layers are present in places, but silt and clay dominate.
The area consists primarily of Tertiary sedimentary and volcanic rocks and Pleistocene glacial and interglacial unconsolidated sediments.
Bald Hill, northwest of Monroe, consists primarily of andesite flows. These blue-green porphyritic rocks weather quite readily to a light grey material with a reddish hue which can easily be disintegrated by hand. At several localities on Bald Hill vertical dikes are exposed which contain andesite xenoliths in a fine grained matrix.

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when

1973-11-05