What causes Continental - Ice Sheet glacier to flow?

Years of unmelted snow piles up. The buried snow is compacted and recrystallized into solid ice. When this ice becomes thick enough it begins to flow under its own weight. The glacier can keep flowing as long as the aculation of snow remains greater than the amount removed due to melting or evapoation etc. As soon as less snow aculates than melts, the end of the glacier will begin to retreat however the glacier itself will still flow forward.