The rocks of the Portland area range in age from late Eocene to Recent. The Tertiary rocks comprise four formations of volcanic origin: the Skamania volcanic series, the Columbia River basalt, the Rhododendron formation, and the Boring lava (Tertiary and Quaternary); the Silver Star granodiorite; the marine Scappoose formation; and two sedimentary formations of terrestrial origin, the Sandy River mudstone and the Troutdale formation.
The Scappoose formation, of late Oligocene and early Miocene age, is the marine sedimentary equivalent, in part, of the upper part of the Skamania volcanic series, the oldest rocks in the area. The Skamania was intruded by the Silver Star granodiorite before the Columbia River basalt was erupted in middle Miocene. The Rhododendron formation, consisting of primarily of volcanic mudflow breccia in this area, was erupted in late Miocene onto a slightly warped surface of the Columbia River basalt on the west flank of the growing Cascade Range in Oregon. The surface of the Rhododendron formation was laterized.
The terrestrial sediments of the Sandy River mudstone and the Troutdale formation were deposited in early Pliocene in structural basins formed by the warping of the Columbia River basalt and the Rhododendron formation. The Boring lava was erupted discontinuously on a post-Troutdale erosional surface.
Late Tertiary and Quaternary deposits include several formations of similar physical character and origin, but distinguished by differences in relative topographic position and degree of weathering. These units are, from oldest to youngest: The Walters Hill, the Springwater, the Gresham, and the Estacada formations. These formations all consist of mudflow deposits and fluvially deposited bouldery cobble gravel. A loessial deposit, younger than Springwater but older than the Gresham, is distributed south and west of the valley of the lower Columbia River, the Willamette Valley, and the Tualatin Valley. These deposits were emplaced by flood waters of great volume that had their source in the sudden release of glacial Lake Missoula. The flood water was successively impounded behind several upstream channel constructions and finally in the valley system of this area by hydraulic damming.
Recent downcutting has modified the terrain, and the deposits of this epoch, including terrace, landslide, and bog deposits, and alluvial deposits of the flood plains are related to this downcutting in large part.
Weathering has produced several profiles of weathering of Quaternary age and two laterite profiles in this area. A low-silica laterite crust formed on the Columbia River basalt represents and end product of weathering lasting from Miocene time to the middle of the Pleistocene epoch. The high-silica laterite, formed on the Rhododendron formation and, locally, on the Columbia River basalt, was produced before the deposition of the early Pliocene terrestrial sediments now represented by the Sandy River mudstone and the Troutdale formation. All Quaternary time was insufficient to produce laterite, but the profile of weathering on the Quaternary deposits is deeper and the rocks are more severely altered with increasing age of the deposit. The younger deposits are but slightly weathered; the older Quaternary deposits are completely decomposed to a saprolite as much as 75 feet in depth.
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