Geologic Overview of the Clackamas Basin

Peter Wampler

OSU

Cascade Mountains

High Cascades

• Young volcanic chain

• Composite volcanoes erupting ash and pumice

• Young basalt flows and cinder cones cover large areas

• Glacial deposits are present near peaks

• Basalt flows may modify drainages.

• Alters hydrology and drainage patterns.

Western Cascades• Produced 6 times the

volcanism of the high cascades

• Collision during formation was “head on” and at a steeper angle

• Comprised of deeply eroded and weathered volcanic rocks

• Prone to mass wasting• Columbia River Basalts

occur with the same geographic area but are more similar to High Cascades in terms of erosion potential

Tectonic Effects on Grains Size

The Willamette Valley Province

• Valley Sediments (filling the bowl)– Recent Alluvium (<1,000 years old)– Holocene (<10,000 years old) Terraces– Pleistocene (>10,000 years old)– Sand River Mudstone– Springwater Formation– Missoula Flood Silts

•Clackamas River is very dynamic

• Cut down from Estacada Surface approximately 140 ft since Missoula Floods

Estacada Terrace