I am told there is some confusion being spread about glaciers and ice sheets flowing uphill. Contrary to the impression some people have got they obey the simple law of gravity, they flow downhill.
But when they are constrained in a passageway either by mountains or other large amounts of ice of course the flow can rise up and over obstacles because of the pressure from the weight of the ice above. The total mass is flowing downhill even if parts of it are going up.
A simple experiment will show it how works, find a short flexible but reasonably stiff string of beads. Provided the are constrained in tube or valley they can easily be pushed over obstacles and will happily bump over ridges on their way downhill.
The key is that their lateral movement is controlled. Now consider the broad dry Bristol Channel during the last ice age, there are no mountains and, as it is at the edge of the ice sheet, no deep layers of ice to constrain any flowing ice. It will follow the downward slope. Try pushing your beads across an incline with a lip at the far edge. Down the slippery slope they will go, not deep into Somerset.
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