Then more snow falls and presses out the air bubbles and turns into blue ice. A mound of fresh snow is composed of 90 percent air space, because snow falls in delicate, six sided, star shaped crystals. The weight of more snow compacts the crystals and makes the snow denser. Warm summer weather melts some of the ice and snow. The resulting melt water percolates down through the snow and further compacts and recrystallizes it into what we call fern, before it becomes very dense glacial ice.
More air is squeezed out as the ice gets thicker.
Crystals at the very bottom of glaciers have bean squashed so much that they don't reflect as much white light as the air-filled crystals did. Because they are denser these crystals reflect only the blue wave length of light. This creates the beautiful clear blue ice seen at the very bottom of glaciers. You need to be roped together if you're on a glacier for safety. We got 12-16 inches.