[Homestead] Ice sheets and Glacial lakes
Don Bowen
don.bowen at earthlink.net
Fri Apr 4 14:18:30 EDT 2008
I recent post on the duration of the ice ages sent me to a book I had on Glacial Lake Missoula. The book did not really address the issue of how long the ice sheet lasted but did mention something like 50,000 years and that over the last two million years there were several ice ages. We cannot be sure because each succeeding age obliterated most of the traces of the last one.
What the book did address and what I found very fascinating is the multiple formation and demise of Glacial Lake Missoula. I lived in Spokane in the early 80s and read some of what was written at the time which was still fragmentary. This book by David Alt brings the research up to date. He mentions some of the controvosary surrounding early works on the subject.
As the ice sheets moved southward down the Percival Trench (the valley where Sandpoint Idaho is located) it blocked off the Clark Fork River. A lake formed behind this ice dam covering much of present day Montana. The surface altitude of the lake could have been as high as 4500'. In the northern edge the lake floated glaciers and ice berg broke off to float around the lake dropping rocks as the went and creating piles of rocks where they grounded and melted.
As the lake level rose it worked under the ice dam which caused the dam to float and crack then burst. This caused a huge flood, Several million gallons per minute and draining the lake in as little as a week. Flood waters cut into the valley south where they hit Basalt and turned west cutting thought the Basalt in the Spokane Valley and flooding much of east central Washington cutting what is not called the Channeled Scab lands. Much of the topology of east central Washington and the Columbia River was formed by these many floods.
As to the number of floods, the best guess is several dozen. As an ice dam broke the relentless push of the glacier dammed the river again and again. Each successive lake was smaller than the proceeding lake as this was near the end of the ice age around 15,000 years ago. The soil of the Spokane Valley was more like a grave parking lot, mostly because of the floods and the later runoff as the glaciers melted.
All very fascinating.
Don Bowen KI6DIU
http://www.braingarage.com/Dons/Travels/journal/Journal.html
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