Where summer melting is less than the winter snowfall, the annual addition of snow results in the growth of a glacier. Snow is "fluffy" but its frilly appendages are broken through blowing and simplified through partial melting and then refreezing, and through compaction, as more layers of snow are added above. Through these processes snowflakes become ice granules called firn. As time passes and slight melting and refreezing plus compaction continues, the firn amalgamates into solid ice - an interlocking network of ice crystals (like an igneous texture). Glacial ice is blue.
Ice is brittle (breaks when under stress) at its surface, but under pressure (under 25 meters of ice) it behaves in a ductile fashion (it flows under stress). The weight of a large mass of ice makes the lower, ductile ice flow outward from the zone of greatest accumulation. Glaciers are flowing streams of ice. The bottoms of glaciers may be frozen to the bedrock if cold enough, or may slide over the bedrock if warmer and lubricated by meltwater.
The upper brittle surface of a glacier forms large open cracks known as crevasses as the glacier bends to flow over a bump in the bedrock.
Where glaciers slide over bedrock, they pick up grit, gravel and even large boulders. These are incorporated into the base of a glacier and grind away at the bedrock over which the glacier flows. Rock flour is fine-grained sediment, mostly silt, produced from the grinding of rock on rock under the weight of a glacier. Embedded rocks scraping along the bedrock produces glacial striations on the bedrock. Glacial striations show the direction the glacier flowed. As glaciers flow over bumps in the bedrock, they grind away, smooth and polish the upstream side and pluck blocks of bedrock from the downstream side. The result is roches moutonées (sheep-like rocks), low bedrock hills that are glacially smoothed so they are lower on the upstream side and drop off more abruptly on the downstream side.
At the end of a glacier, where it is melting as fast as it is being supplied by ice from upstream, large quantities of unsorted sediments (silt, sand, gravel and boulders) called glacial till are heaped into terminal moraines. Lateral moraines develop at the sides of glaciers where rocks tumble down from steep valley walls. Medial moraines form from lateral moraine material where two glaciers merge.
During glacial epochs like the last few million years, continental ice sheets advance and retreat from the polar regions over time spans of tens of thousands of years. These glacial cycles are caused by variations of the Earth's orbit around the sun which changes the amount of solar radiation coming into the Earth at high latitudes during the summer. Continental ice sheets leave behind a number of characteristic features. Long, winding terminal moraines show the position of the farthest advance of the ice sheet. Drumlins are elongated mounds of glacial till deposited and molded by the flowing glacier. They are oriented in the direction of glacial flow and are typically somewhat higher and wider on the upstream side. Eskers are sinuous low ridges of sorted sediments formed by subglacial streams. Kettles and kettle lakes are depressions formed by the melting of ice blocks deposited within masses of sediment as a glacier melted. Beyond the terminal moraine lies an outwash plain, covered by sorted stream deposited sediments transported by meltwater streams from the end of the glacier.
Alpine glaciers descend from high, cold mountain peaks cutting deep U-shaped valleys. At the head of the glacier a deep bowl, called a cirque, is excavated by the glacier. Many cirques are filled by small lakes called tarns when the glaciers melt. Cirques have a steep headwall that formed as the cirque glacier cut downward on the side of a mountain. A horn is a faceted mountain peak, with steep headwalls carved on several sides by cirque glaciers. An arête is a high pinnacled ridge carved by glaciers on both sides. Hanging valleys, many with beautiful waterfalls, are formed where smaller tributary glaciers once fed into large, deep-cutting, glaciers.