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(Note: this is from U.S. point of view)

I keep hearing how the glaciers "moved South" during the last ice age. But I thought glaciers moved downhill.

I can see that the glaciers formed farther and farther south as the temperature fell. But then they presumably moved downhill, following the landscape beneath them.

However: assuming the glaciers were much thicker to the North, then the (hydrostatic?) pressure beneath them would be higher than the Southern glaciers; maybe that pushed everything South.

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    $\begingroup$ I assume this question specifically refers to the Laurentide Ice Sheet? Because the glaciers in California's Sierra Nevada did move downhill. Since ice flows like a very high viscosity liquid, I think the explanation from your last paragraph applies, but at this time I don't have literature references to back this up. $\endgroup$
    – njuffa
    Dec 20 '20 at 23:10
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    $\begingroup$ Yes, Laurentide. $\endgroup$
    – Daniel
    Dec 21 '20 at 2:11
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    $\begingroup$ Continental-scale ice sheets don't flow downhill, unless you consider the glacier itself to be the hill. They flow outwards under the pressure of accumulated ice, which is typically several km deep. $\endgroup$
    – jamesqf
    Dec 21 '20 at 3:57
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    $\begingroup$ If you google for "Laurentide Ice Sheet flow" and click "images" you will find maps with arrows. It looks complicated - some seem to show flow towards the continental divide in northern Alberta/BC. Note that there can be isostatic depression of the land due to the weight of ice, so "downhill" was not the same as now. $\endgroup$ Dec 21 '20 at 5:09
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    $\begingroup$ This 2015 review article On the reconstruction of palaeo-ice sheets: Recent advances and future challenges is worth a look, if only for the pictures! $\endgroup$ Dec 22 '20 at 2:25
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Did glaciers move South or move downhill?

You are posing this as an either-or question. The answer is "Yes."

The highest points above sea level in Manitoba, Minnesota, Ontario, Michigan, Wisconsin, Iowa, and Illinois are respectively 832 meters, 701 meters, 693 meters, 603 meters, 595 meters, 509 meters, and 376 meters. Central Canada and the United State's upper midwest are very flat.

Even the eastern parts of the US and Canada are relatively flat compared to the huge ~4000 meter thick ice dome centered roughly on Hudson Bay. Every direction was downhill from the peak of this ice dome. The ice flowed to the west to meet ice streams from the Rocky Mountains (and then turn north or south), to the north toward the Arctic Ocean, to the east toward the Atlantic Ocean, and to the south to meet warmer temperatures. The ice at the southern edges was much thinner, but it still was a downhill flow.

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Glaciers flow downhill and outwards. Outwards means if you form a glacier on flat (or with a round body, gravitationally equipotential) ground, then its weight will eventually push it outwards in all directions on the ground; to stop this movement in any direction you need an uphill slope to oppose the outward push from the mass of ice.

To the extent that glaciers flow outwards those in the Northern Hemisphere also flow southwards (in the Southern Hemisphere it would be northwards) on a hemispherical scale, as this would be outwards from the pole. But another factor also contributes to the southward/northward advance in the Northern/Southern Hemisphere. If the glaciers are growing because of overall cooling around the globe (or at least around the hemisphere) then the region in which ice can accumulate over time instead of all melting each spring advances to lower latitudes. Thus the glacial area grows into lower latitudes as the global or hemispherical climate cools through precipitation as well as through the hemisphere-wide outward flow.

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I think that it’s a language issue.

When people talk about glaciers “moving south”, they don’t mean that any specific glacier literally moved south. They mean that the general area in which glaciers exist used to extend more south than it used to.

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