The Late Cenozoic formations in the lower Columbia River drainage basin constitute one of the best records of the terrestrial history of Western North America. The main events within the Cascade Mountains and westward to the Pacific Ocean, which culminated in the Cascadian Revolution, are:
Columbia River basaltic lavas were erupted from fissures during the middle Miocene upon a low-lying terrain of older Cenozoic sedimentary and volcanic rocks. Marine sediments of the Astoria formation were deposited just prior to extrusion in what is now the seaward part of the valley. The flows interfinger with beds which in general overlie the marine members of the Astoria formation.
Limonite beds and associated clastic sediments were deposited in depressions locally developed on the lava plain.
Additional lavas were extruded in the bog areas and perhaps on other parts of the lava plain.
While the Columbia River basaltic terrain lay near sea level, the uppermost basalt was extensively laterized to ferruginous bauxite. The Columbia River had established essentially its present course at or near the margin of the lava plain, but erosion prior to deformation was of little consequence.
Northerly trending open folds were produced in the basalt with those near the axis of the Cascade Range showing the greatest effect of the compression. Contemporaneously, anticlinal highs were modified by erosion and the lower part of the Troutdale formation was deposited in downwarps along the Columbia River while similar sediments were laid down in tributary valleys. Intercalated pyroclastics and flows herald the start of widespread Cascade volcanism. Basic lavas and pyroclastics were erupted from numerous vents including the major cones. All these events probably took place in the first half of the Pliocene.
Additional sediments were deposited as sea level rose in later Pliocene time. Most of the present valley areas were covered with fluviatile sediments and the rivers established their present courses on or from such fills. Most of the Cascade volcanic activity had apparently taken place before the Portland Hills silt member of the Troutdale was deposited in the late (?) Pliocene time.
Differential uplift began in late (?) Pliocene time and has continued to the present. The cutting of the Columbia River Gorge is the result of uplift which near the axis of the Cascade Range has been more than 2500 feet. Uplift in the Coast Range has been somewhat less.
Downcutting by the Columbia River during the (?) early Wisconsin, as a result of a eustatic lowering of sea level, amounted to approximately 600 feet. Deposition of the Portland gravels with their glacial erratics and contemporaneous sediments occurred with the ensuing rise in sea level.
Subsequent uplift in the Portland area has amounted to about 400 feet.