Cliff Ollier has written a text about the mechanisms underlying the flow of glaciers. James Hansen uses very naive models of the flow which is the main technical reason for his catastrophic predictions.
Ollier argues that the most important process behind the flow is "creep" that dominates whenever the stress exceeds a limit (yield stress). This motion explains why ice crystals at the lowest end of glaciers grow faster than others. Other aspects of glacier dynamics are discussed, too.