South of the Puget Lowland and Cascade Range, the Portland Basin marks the northern terminus of the Willamette Lowland in Oregon. Located in western Clark County, the northern portion of the Portland Basin is known for its low topographic relief. The Columbia River has carved a channel through the current-day basin near Vancouver, WA.
Around 20 million years ago, compressional forces initiated the formation of a depression that trends northwest to southeast. Eventually the rocks making up this basin buckled under the pressure. The valley continued to deepen as faults along the north and south margins of the basin formed. These tectonic stresses applied over long periods of time have eventually formed a large basin, into which volcanic and sedimentary rocks were deposited.
Columbia River Basalt Group
During its early formation, about 16 million years ago, the basin was flooded with voluminous basalt and basaltic andesite flows from the Columbia River Basalt Group, which originated in eastern Washington and Oregon. At the northern and southern edges of the basin, these flows are exposed. Within the basin itself, however, the flows are now buried more than 1,000 feet below the surface.
At the end of the Miocene and continuing until about 4 million years ago, the basin filled with sediments of the ancestral Columbia River. Named the Troutdale Formation, these deposits can be divided into two general parts: a lower gravel-rich section containing pebbles and cobbles derived from the Columbia Basin and the Okanogan Highlands, and an upper section that contains glassy volcanic sands. The sands owe their origin to Simcoe Volcanics flowing into the Columbia River, cooling extremely quickly, eroding, and then redepositing downstream.
Deposition of the Troutdale Formation was followed by a period of volcanism between 0.6 and 0.13 million years ago, which produced the Boring Lavas. These basalts aren’t exactly entertaining, but they were really named for its type locality near Boring, Oregon.
During the last ice age, glaciers covered much of North America and blocked a fork of the Clark River in western Montana. The water continued to flow and build up behind a giant wall of ice 2,000 feet high. A glacial lake called Glacial Lake Missoula formed behind the ice and is believed to have been the size of modern-day Lake Michigan. This ice dam broke repeatedly between 12,700 and 15,300 years ago. Catastrophic floodwaters flowed down the ancestral Columbia River through the Portland basin. These high-energy flows carved much of the landscape in southern Washington. The water ponded in the Portland Basin and backwaters caused the deposition of well-sorted sand, clay, and gravel.
Regional map of Pleistocene glaciers, glacial lakes, and outburst floods.