The tremendous weight of the glacier makes the crystals of ice slowly form one atop another. Then the layers begin gliding one over another. The thicker a glacier is the faster the glacier moves because the heavier it is it causes it to move faster. The middle of a glacier moves faster then the outside. The top of a glacier moves faster then the bottom.
Temperature is another reason that affects the speed of a glacier.
Slowly but with irresistible force they cut and carve the land. Some glaciers can move up to 100 feet a day. In 1827 a Swiss scientist built a hut on a alpine glacier. When he returned 3 years later he found out the hut had moved more then 100 yards downhill. Some glaciers move so slowly that you might not notice that they are moving for a long time.