Much of Puget Sound is nothing but Pleistocene - with Vashon glacial deposits on the surface and older glacial and interglacial sediments peaking out from the lower portions of some of the bluffs. But at Rocky Point (2009), on the west side of Whidbey Island, and in Skagit Bay (Craft Island 2011, Hope Island 2012), the Mesozoic re-emerges from the basement to form rocky islands and headlands.
Deception Pass, separating the north end of Whidbey Island from Fidalgo Island, is a topographic gap in these older metamorphic rocks, which make up much of the San Juan Islands to the north and west.
Deception Pass moves a lot of water, including much of Skagit Bay and Saratoga Passage and the currents are fast. Steep rocky cliffs plunge into deep water in two channels, split by Pass Island. But there is sediment moving at depth and USGS work has shown a submarine delta of sorts west of the entrance (I know I've seen some bathy from here - but can't find it online). There are pocket beaches on both the Whidbey and Fidalgo sides of Deception Pass, probably consisting of sediment derived from the erosion of overlying glacial drift, although the ones on the south side may also include sediment that has made it around the corner from the sediment rich beaches on northwest Whidbey.