"Scraping and Grinding"
The "Bedrock" Exposed in Deception Pass State Park
From the northern edge of Whidbey Island, across Fidalgo Island and further on to the San Juan Islands, the geologic framework changes radically. Most of the terrain is underlain by ancient, hard crystalline rock, referred to generally as "bedrock." It is exposed here, whereas on Whidbey Island it is buried by glacial sediments. Deception Pass is the boundary between where the hills and valleys to the north were eroded away by the continental glaciers to where the glaciers dropped the sand, silt and boulders when they began to melt and retreat. The only other bedrock outcrop is along the western shoreline of the Naval Air Station where it has been brought to the surface by reverse faults.
For a quick visit to the best exposures of these older rocks, stop at Deception Pass Bridge. Cliffs both above and below the bridge are cut into 150-million-year old submarine lavas and associated sediments that were deposited on the sea floor. These deposits were deformed and metamorphosed during collisions between oceanic and continental plates. They were carried to great depths where high temperatures and pressures altered their textures and were, eventually, brought back to the surface. These rocks have been altered from their original forms, cut by faults, and are.......a mess. Unraveling the puzzle of where these rocks originated and how they got here has taken a lot of field and lab work by geologists.
Large glacial "erratics" are bedrock boulders that fell onto glacier surfaces as they moved south. For example, the "Big Rock" in Coupeville was likely plucked from bedrock near Deception Pass (13.5 miles north). The 38 x 60- foot Waterman erratic, in the Saratoga Woods near Langley, is 30 miles south of a greenschist source on Fidalgo Island.
Evidence of glacial erosion of the "basement" rocks can be seen along the trails over Goose Rock, east of Highway 20 and southeast of Deception Pass Bridge. The rock has been smoothed and scraped by moving ice laden with sand and rocks. Think of coarse sandpaper. These glacial striae preserve evidence that the glaciers were moving from northeast to southwest.
Glacial striae on metamorphic rocks exposed on Goose Rock, near Deception Pass. The striae indicate that the glaciers were moving from northeast to southwest.