The oldest strata come from the Double Bluff Glaciation (150,000 - 200,000 BP), and the youngest—at the very top of the bluffs—is composed of drift from the Possession Glaciation (~70,000 BP). Between these two layers is the Whidbey Formation, an interglacial layer composed of sediments laid down by lakes, streams and rivers within the Puget Sound basin, before it was inundated by the Pacific Ocean. The Whidbey Formation composes the majority of the visible bluff face and consists of poorly sorted silt, clay, and sands, pebbles, and compressed slabs of peat. Mammoth teeth and tusks have been found eroding from the bluff.