During the Eocene Period, some 50 million years ago, sediments collected across what is now western Washington. The majority of these deposits were continental in origin, eroded from pre-Cascade uplands to the east; some, especially across southwestern Washington, were marine in origin, deposited in shallow seas as ancestral whales were first returning to the ocean.
Today, these Eocene sediments underlie Puget Sound and the urban areas that hug its shoreline. Continental Eocene deposits, both sedimentary and igneous, also comprise the Olympic Mountains and a mix of marine and terrestrial Eocene sediments have been folded to create the Coastal Range, further south; nearing the Oregon border, younger Miocene deposits also appear in this latter Range.
The modern Cascades, volcanic in origin, are composed of much younger rock, forming from the Miocene (about 20 million years ago) to the present day. The major stratovolcanoes (Ranier, St. Helens, Adams) rose during the Pleistocene (within the last 2 million years) and, as became painfully evident in 1980, are still evolving. Of interest, the Northern Cascades are composed of much older Jurassic magma, intruded some 150 million years ago but lifted and sculpted as the younger Cascades developed.
