Hard to keep in mind that solids can flow, like salt rising slowly

I may well be wrong here, but I believe that in glaciers the motion is by extrusion -- like squeezing toothpaste from a tube.

That is: ice is somewhat plastic under pressure. A glacier's bottom ice is under tremendous pressure from the weight of the ice and snow above, and that pressure extrudes the ice outward, just as a ball of clay will spread when you press down firmly upon it.