South of the Puget Lowland and southwest of the southern Cascade mountains, the Portland Basin marks the northern terminus of the Willamette Lowland of Oregon. In Washington, the northern portion of the Portland Basin is characterized by the low topographic relief of western Clark County.
Both upstream and downstream from Vancouver, at the edge of the basin, there are exposures of Columbia River basalt. Within the basin itself, the basalt units lie more than 1,000 feet below the surface. Starting during the Miocene and continuing through the Pliocene, the basin was filled by sediments of the ancestral Columbia River. Named the Troutdale Formation, these deposits can be divided into two general parts: a lower gravel section containing pebbles and cobbles that were derived from the Columbia Basin and the Okanogan Highlands, and an upper section that contains vitric volcanic glass (hyaloclastic) sands. The hyaloclastic sands owe their origin to Cascade Range Simcoe Volcanics flowing into the Columbia River, explosively quenching, and then being redeposited downstream in the Portland Basin as sand. A volcanic breccia subunit of the Troutdale Formation, representing a lahar, has been mapped near Woodland, Washington.
Deposition of the Troutdale Formation was followed by a period of Boring Lava volcanism 2.6 to 1.3 Ma. Centers of extrusive activity have been documented around the margins of the Portland Basin. This volcanism was associated with faulting and structural deformation of the Troutdale Formation and further depression of the Portland Basin.
Lastly, as the glacial Lake Missoula catastrophic floods burst out of the Columbia River Gorge 12,700 to 15,300 years ago, the waters ponded in the Portland Basin. Backwaters caused the deposition of well-sorted sand, clay, and gravel.
The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.