A rather large amount of the terrain we lay our eyes on in the mid-to-high latitudes have been shaped by glaciers. A major clue is to this action is when we see "U-shaped" valleys, or scoured "circs" - like someone took a giant spoon to the side of a mountain made of Extreme Moosetracks ice cream. These tell a practiced eye that something big and remorseless has ground everything smooth. Most glaciers are not small things, either. I have been in a fjord in Norway that has scoured edges 3,000 meters up from the current water-line! For American readers that represents a moving mass of ice about two miles deep. The huge weight of these glaciers at their height 10,000 - 16,000 years ago, but now removed, has caused parts of Scandinavia to uplift - float up - by as much as a meter in a century.
Q:
Hi, I live <in> Minnesota which experienced substantial effects of glacial movements during the ice ages. My question is: How were the gravitational forces sufficient to move the glaciers south into the upper Midwest given the current elevations above sea level of Hudson Bay (400 ft), Duluth (1430 ft), and Minneapolis (840 ft). Thanks Jim T.
A:
You can't think of glaciers as slow-moving water - they are plastic, with high viscosity. Precipitation moisture falls and "pancakes" instead - compresses, forming glaciers, with the center of the precipitation building up steadily, and the edges moving outwards. If you pressed down on the center of a still-fluid core of a pancake (before it was cooked through), it would extend or "moosh out" sideways - this is a displace-and-inflate behavior we also see in lava in Hawai'i. If the pancake filled the entire griddle or pan, and there was confinement at the edge or lip of the griddle (the analog of the broad increased elevation at Duluth), then the plastic material would rise above and flow over this barrier. If the elevation-increase is an isolated mountain, the glacier will instead slowly move around it on both sides - but still lap up locally at its edges.
Keep in mind that behind the continent-scale glaciers was just more glacier all the way back to central Russia - so the ice front could only move southward in North America. And believe me, nothing epitomizes the word "inexorable" quite like a glacier on the move.
Hmmm. It's early in the morning, and I must be still thinking of breakfast to come up with a pancake metaphor.
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