How a Glacier Works What is a glacier?  In places where the summers are not warm enough to melt the snow that falls each winter, the snow can accumulate year after year, eventually forming huge sheets of ice called glaciers. In places where more snow falls during the year than melts, the snow builds up, sometimes thousands of feet deep.  The compacted snow turns to ice, forming a glacier. When the ice becomes thick enough­—about 150 feet—pressure at the bottom becomes so great that this ice deforms and flows plastically. What Can Glaciers Do? Sculpt the Landscape Massive and slow-moving, glaciers are capable of pushing millions of tons of earth before them as they flow. Carry Rocks and Sediment Carry Rocks and SedimentAs glaciers flow, they pick up rock and sediment by the ton, carrying it sometimes hundreds of miles from its original source. Deposit Sand and Gravel When glaciers melt, the water that flows out carries with it many tons of sediment previously frozen in the ice.