Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. Firn has a density greater than 0.55. Advance occurs when the rate of glacier flow downvalley is greater than its rate of ablation. Advances are characterized by a convex-shaped terminus.
Glacial drift is the loose and unsorted rock debris distributed by glaciers and glacial meltwaters.
Collectively, these deposits are called glacial drift . Till refers to the debris deposited directly by the glacier. Rock debris rolls off the glacier edges and builds piles of loose unconsolidated rocks called glacier moraine . Lateral moraines form along the side of a glacier and curl into a terminal moraine . Its presence turns lake water aqua blue or brown, depending on its parent rock type.