This report from 1973 describes the geology of southern Snohomish County.
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.
The area consists primarily of Tertiary sedimentary and volcanic rocks and Pleistocene glacial and interglacial unconsolidated sediments.
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. Its vertical extent is from well below sea level, as indicated by well records, to about 200 feet above sea-level. There may be sequences up to 100 feet of almost pure clay. A good exposure of the thick clay unit is located in an active clay pit in Section 21, T.28N.,R.5E. 5 miles southeast of Everett. The exposure shows the bedded nature and blue color that is characteristic of the Whidbey Formation. There are other good exposures of the Whidbey clay located along the High Bridge Road west of Monroe. The known extent of the Whidbey Formation in the map area is about 5 square miles.