All of us are enthralled by the large, High Cascade volcanoes such as Mount McLoughlin and Mount Shasta. But did you know there were much older but just as impressive volcanoes, now humbled by erosion, forming the northeastern ridges of our valley?
These rocks, lying west of the High Cascades, are known collectively as the Western Cascades Volcanic Group (or Little Butte Volcanics) to which Mount Baldy, Grizzly Peak and Pilot Rock belong.
These volcanic rocks extend from southern British Columbia to the eastern part of the Shasta Valley. In our area, rocks range from about 21 to 38 million years old (late Eocene to Miocene), though their eastern extent is covered by an extensive blanket of High Cascades volcanic rocks.
Rafters jouncing white-knuckled along the Klamath River, which cuts through the “blanket,” will see younger Western Cascades rocks extending almost to Klamath Falls.
The Western Cascades consist of a complex array of overlapping volcanoes displaying a wide range of compositions. Very fluid to sluggishly flowing lava and volcanic ash formed during periods of intense eruptions. However, most rocks consist of broken fragments embedded in a finer matrix (breccia) deposited on volcanic flanks as muddy debris flows or rock avalanches. Exposures of Western Cascades rocks are seen along Highway 140 to the plateau just beyond Lost Creek Reservoir, Highway 66 to Howard Prairie, and Interstate 5 from the Siskiyou Summit to the state line.
The Siskiyou Summit is a great place to observe shallow stream deposits on the flank of an old volcano. Look at the base of white volcanic tuff (ash) deposits to see dark, flattened tree trunks and carbonized leaf fragments.
The West Coast of North America was as active then as it is today. The earth's crust was stressed, producing earthquakes that broke and tilted early volcanic deposits, subjecting them to erosion. New volcanic material was deposited on their humbled remains. The upper part of the older Payne Cliffs Formation, discussed in an earlier article, heralded the onset of volcanism, yet it, too, was broken and eroded, especially to the south, producing a chaotic and non-uniform history.
Do we see any easily recognizable, Mount Fuji-like volcanoes? Sadly, no. The Western Cascades rocks have been multiply eroded, buried by younger volcanic rock to depths of at least five to possibly seven kilometers, tilted toward the east, and broken by faults, obliterating most traces of individual volcanic centers. Like Indiana Jones, they've had great adventures but a very hard life!
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at firstname.lastname@example.org.