The Crater . . .
This photo of Mt. Powell (10,168 ft.) near Deer Lodge in western Montana is named "The Crater" (on maps), but its origin has nothing to do with volcanic activity, or asteroid impacts. Instead it is an impressive cirque, carved by a glacier that once flowed from Powell’s northeast slope, down toward the valley of the Clark Fork River. According to geology maps, the strange flow-shaped mass of rocks near the bottom of the cirque was left by a “rock glacier”. Apparently during the final decades of Mt. Powell’s glacier, there were more rocks than ice in the mix. Eventually even the ice between the rocks melted away, and the rocks were left without a “ride”. From the summit, the deposit looks like a fluid blob of rocks, but without the matrix of ice the rocks are no longer flowing.
I thought glaciers were made of ice . . .
If glaciers can be described as rivers of ice, then rock glaciers can be described as rivers of rock and ice. They can be spotted in relatively arid (dry) alpine (mountainous) environments as steep-edged, tongue-shaped, talus-mantled lobes coming off the slopes of valley walls or peaks. Rock glaciers have steep edges because there is ice inside them, holding all the rocks in place at an angle that is steeper than would otherwise be natural for a pile of rocks. This ice inside of rock glaciers can either be a solid core that perhaps was a glacier back before it was buried in rock, or it can be frozen between rocks as a sort of glue that formed as rain water and snow melt ran between rocks and then refroze.
