Last week was Glaciers in North America which led me to a line of thinking. Back when I was in high school, we were taught that the great glaciers came down into the Willamette Valley. This was evidenced by the ‘erratic rocks’ found throughout the valley. Then, we would see pictures of Yosemite and the European Alps showing u-shaped and hanging valleys. Of course, I would mention that I had never seen a u-shaped or hanging valley in or around the Willamette Valley. The teacher would reply that erosion had changed the valley floor; which I never believed!
In the late ‘70’s and into the ‘80’s documentaries started showing up about plate tectonics, glaciations, and the Missoula floods and I was hooked. All of a sudden I was seeing explanations for the Cascade volcanoes, the earthquake-volcano connection, how glaciers form and move, and best of all --- where those erratic rocks came from!
Turns out during the last ice age the glaciers did not come south far enough to enter the Willamette Valley. The Willamette River flows north, like the Nile, so if a glacier came far enough south to plug up the Willamette there would have been evidence of a lake or a new river bed flowing towards the east or west. And there is no such evidence.
|Erratic Rock State Park|
The cause for the erratic rocks ---- The floods from Lake Missoula! Lake Missoula formed, during the last ice age, behind an ice dam in present day Montana. The lake was so big it reached up into Canada. When this massive amount of water broke the ice dam it flowed over the ‘Scablands’ of Eastern Washington, scouring the soil down to the bed rock; over ‘Dry Falls’ a waterfall that out did Niagara Falls by a lot; following the current Columbia River course to the Columbia River Gorge, where it forged through the narrow canyon; on to the wide mouth of the Willamette River and on to the Pacific Ocean.
When the water, full of the soils it had just lifted off as well as chucks of ice from the ice fields, reached the Willamette River it flowed up river; such was the force. As the back water sat in the valley the suspended solids began to fall out. In the Portland area the soils are sandy and some areas are downright rocky. Heaviest solids fall out first. The area down to about Salem has wonderful silty soil. South of Salem to Cottage Grove has increasingly clay soils.
The erratic rocks were carried on the ice chunks. They are found on a shelf around the edge of the valley. If you ever want to see one of the rocks, try Erratic Rock State Park off Hwy 18 between McMinnville and Sheridan. There is another on Brownlee Road, between East Ellendale Road and Orrs Corner Road, just outside Dallas.
(Dallas was named for the VP of President James Polk. It is the county seat of Polk County. You are welcome.)