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.