Originally Posted by
TimmyT
Glaciologist checking in here.
. . .
Because the summertime melt is high during retreat, there is a lot of "excess" water that carves the landscape. We almost never see the growth phase of the ice sheet on the land surface because the retreat phase is so strong and it's the last thing to happen.
I hope this helps.
Very much, thanks. One of the hallmarks of glaciation near my neck of the woods is the U-shaped valley with hanging waterfalls. The Finger Lakes region has some excellent examples, but there are less striking ones along the Niagara Escarpment. In more mountainous regions like the "Canadian" Rockies (defined as extending south to Marias Pass, Montana, so including Glacier Park) and the western shore of Newfoundland they make spectacular landforms. These started out as V-shaped valleys but we see them today as U-shaped. I wasn't around to eye-witness any of that. So were the valleys scoured out by the weight of the advancing ice pushing the rocks ahead of it? ... or flooded out by the torrential meltwater during retreat? Until the ice retreated enough to open the St. Lawrence valley, "Lake Iroquois" (now Lakes Ontario, Erie, and part of Huron) drained out through the Finger Lakes to the Atlantic via the Susquehanna River. That's a heck of a lot of water. I understand there is some controversy around the shaping of these valleys and I'd value your thoughts.