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