Glacial Lake Missoula was up to 2000 feet deep and covered 3000 square miles of western Montana. Catastrophic failure of the Clark Fork ice dam released 500 cubic miles of water at a rate 10 time the combined flow of all the present-day rivers on earth.
The Ice Dam was here at the mouth of the Clark Fork River. View toward the Green Monarch Mountains where five or more spillways pass through ridge.
|View of the Green Monarchs from the Drift Yard. Pole Creek spillway is a little to the left in this photo.|
|Pole Creek Spillway in the center, just above the trees.|
|Closeup of Johnson Creek. It was covered with ice. You can see a scoured rock.|
|On the bridge at Clark Fork|
Then we drove to Castle Rock. We drove over what is the remains of gigantic ripple marks to stop here. Castle Rock is part of a dry cataract. You can see glacial polish and striations in the rock.
Next, we drove to the Dry Creek Gravel Pit. There is evidence here that the glacier moved east towards Thompson falls and deposited the rocks as it moved. (See below) The aggregate here is cemented and the pit walls must be ripped before it can be excavated. This gravel probably was deposited in the latest glacial time from the ice margin into Lake Missoula.
The late glacial ice advanced upstream almost to Thompson Falls, Montana. The late phases of Lake Missoula drained slowly enough to leave these deposits preserved.
I am not much of a geologist so when the conversation includes topics such as the Wallace Formation, the Prichard formation and granitic magmas, my mind wanders. I start thinking about where to transplant the new lily in my garden, what notes and chords I should play when the band plays Georgia, or I take strange photos that have nothing to do with the field trip.
Yes, I learned something new. It was pounded into my brain something like a glacier slowly rolling over the top of me and depositing little bits of information here and there.
Dang. I've got to get back to my garden and my music. I'm starting to lose it.