When a glacier reaches very large thickness, which is usually 18 metres thick it becomes very heavy. This causes it to deform which causes it to move. The glacier moves because of the gravity applied on it and the weight of it.
When the glaciers move they move very slowly down valleys, across plains or spread into the sea. When the underside of the glacier moves slower because of the friction as it slides and the top moves faster. Glaciers can either retract or go forward. Glaciers retreat or go forward depending on the amount of accumulation of lack of it. It also depends on the position of the snout. The glacier can retreat quickly or slowly.

-Logan   

Glaciers form when there is snow that remains year round. Shortly, enough snow builds up and it turns to ice. Every year, new layers of ice form which now compresses and buries the remaining layers from the years before.
As the snow falls it goes from flakes to grains. The compression that the new snow applies on the remaining snow causes it to re-crystallize. The re-crystallizing forms smaller grains of snow. The particles rearrange themselves to a circular form. These things make the snow to compact and causing it to increase in density.
After around two years the snow forming the glacier turns into a firm. A firn is the state snow and glacier ice. At this state the glacier is half as dense as water.
As the glacier melts, water trickles down through it and refreezes between the crystals.  Over time vast ice crystals become so compressed the air pockets between them are very tiny. Inside the glacier the crystals can grow many inches long.

-Logan