Glaciers form when show piles up in mountain hollows, called cirques. The snow is gradually compacted into a white substance, halfway between snow and ice. As the pressure increases, the ice crystals become closely interlocked, while water, seeping down from above, refreezes and binds the crystals together. Finally, the compacted snow is pressed into clear blue ice.
Exactly how ice flows downhill ins’t completely clear, but is does seem that stress and pressure release molecules of water between the ice crystals, so that they slide over each other. Melting and refreezing of the ice underneath the glacier also helps it to slide slowly downhill.