The Trail Blazers, Pioneer Square and 583, 776 residents are not the only things that can call the city of Portland home. Beneath the frequently walked streets of the ever-busy town lies impressive geology. This city, situated at the union of the Columbia and Willamette River populates the western area of the Portland Basin. This forearc basin lies in between two ranges: the Coast Ranges and the Cascade Ranges.

This basin is also chained by the Cascade Range to the east and by the Tualatin (a southwestern suburb of the Portland area) Mountains to the west; it is navigated by the Colombia River (this river has had a heavy influence on the basin’s geology.) and has been growing since the Miocene.

The Tualatin Mountains act as a separator between the Portland Basin and the Tualatin Basin that is situated to the West. (The Tualatin Basin is known to be flat and contains a small amount of folded and faulted highs in the middle.) In the later part of the Pleistocene there were strong east winds that carried through the Columbia River Gorge. Along the way it gathered glacial silt from the Columbia River floodplain and delivered it as loess throughout the mountains. This delivery was the cause of the formation of the Portland Hills Silt.

The strength of the Portland Basin (noted to be unusually strong) may be due to the fact that the oldest rocks exposed in the city are the basalt flows as well as the marine sedimentary rocks that are from the Eocene Basalt of Waverly Heights. These rocks have closely related chemistry and ages to that of the Tillamook Volcanics in the Northwest Oregon Coast Range.

Around 3 to 2.4 million years ago eruptions consisting of basalt rich in olivine and basaltic andesite occurred throughout the Portland Basin, ultimately resulting in the formation of the Boring volcanic field. A good portion of these eruptions produced either plugs, flows or small shield volcanoes that took up a majority of the area.

The Portland Basin, along with the Willamette Valley and Tualatin Basin were flounced due to repeated glacial outburst floods of colossal proportions that went by the names of Bretz, Missoula and Ice Age Floods. These floods made their marks 23-15 thousands of years ago and helped to drastically reconfigure the Portland area blueprint.

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Portland Basin: a forearc basin lying between the active volcanic arc of the Cascade Range and the Coast Ranges

Coast Ranges: comprised of marine sedimentary rocks from the Eocene (time range here) and Miocene (time range here) as well as Eocene extrusive basalt that was deposited on the Siletz terrane that had been accreted.

Cascade Ranges: a collection of a sequence of arc volcanic rocks from the Eocene to Holocene and embellished with a lineup of Quaternary volcanoes.

Tualatin Mountains: A straight and narrow range that have a sharp yet fault-bounded edge.

Loess: Clastic/slit sized sediment.

Portland Hills Silt: With the presence of paleosols, there is indication that this silt accumulated during not only one glaciation-the silt is at least 30m thick.

Northwest Oregon Coast Range: Mountains formed from igneous rocks, basalt flows and sandstone around 40 million years ago during the Eocene age.

Boring Volcanic Field: Centered in the Portland Basin, merging to the east, and is constituted of 80 smaller volcanoes.

Willamette Valley: Located in the northwestern part of Oregon, this region is surrounded by mountain ranges to the west, east and even south. The valley floor is not only broad and flat but also fertile thanks to the conditions of the Ice Age.