Portlandia: Very Brief Geology Summary

A little brief summary of Portland geology.

The west side of the city has a fault line that uplifts the west hills above the low lands. Movement on this fault would be a disruptive event.

Much of the city is built on a broad ice-age flood bar deposit. As ice-age floods surged out of the Columbia River Gorge to the east the flow widened and lost velocity and thus a sequence of sand and gravels were deposited. The Columbia River and the Willamette River have subsequently incised down through these deposits. Ice-age flood deposits also cover much of Vancouver, Washington north of the Columbia. A few road cuts along I-5 north of Vancouver in the Kalama area provide cross sectional views of the flood deposits.

To the east of the city and scattered within the city above the flood deposits are the Boring Hills. Several of these hills area cinder cones with craters and lava flows. These small volcanoes are as recent as about 100,000 years old.