Most glaciers that exists today are remnants of the last glacial period from 1,800,000- 11,000 years ago.  Glaciers form in areas where the air temperature never gets warm enough to completely melt the snow. After snowfall, some or all of the Snow will melt when it comes in contact with warmer  ground temperatures. As the air temperature drops, the melted snow refreezes, turning into small ice granules called firn or neve. When additional snow layers are accumulated on top,  the firn is compacted underneath. When accumulation reaches 150 feet deep,The weight and the pressure cause lower layers to be crystallize into solid ice. As the years pass,  accumulates and the slab of ice grows steadily thicker. Eventually the mound of ice becomes too massive to sit still, and gravity pulls the ice downhill. Once the ice begins to move it is considered a glacier!!