Main Topics: Module 7

All piles tend to spread under their weight.
A glacier is a pile of ice and snow that forms where snowfall exceeds melting long enough to make a pile that is big enough to spread under its own weight.
A glacier takes water (as ice) and sediment from the accumulation zone (where snow accumulates faster than it melts) to the ablation zone (where melting, also called ablation, exceeds snow accumulation) or to calve icebergs that float away in surrounding oceans or lakes to melt elsewhere.
A glacier flows in the downhill direction of its upper surface (where ice meets air), even if that means the bottom flows uphill like pancake batter spreading across the bumps on a waffle iron.

A glacier moves by deformation within the ice, and if the bed is warmed to the melting point, by sliding over the glacier's bed or deforming the sediment there.
Most deformation in the ice of a glacier happens near the bottom where stresses are highest, but the top of a glacier moves fastest because it rides along on the deeper layers.
The ice of a glacier deforms under stress because the ice is almost hot enough to melt, even though the ice feels cold to us.

Glaciers erode by plucking rocks loose, sand-papering (abrading) the bed, and through the actions of subglacial streams that occur beneath some glaciers.
Glaciers with thawed beds, especially those with surface meltwater reaching the bed, change the landscape more rapidly than is typical for landscapes shaped by non-glacial streams, wind, or mass movement.
Abrasion (sandpapering) under ice makes striae (scratches) and polishes rock.
Abrasion smooths the up-glacier sides of bedrock bumps, while the glacier plucks blocks loose from the down-glacier sides of bumps.
Glaciers erode valleys to give them “U”-shaped cross-sections, often with the floors of side valleys flowing into the main valley left "hanging" above the floor of the main valley; streams make “V”-shaped valleys without hanging valleys.
Glaciers gnaw bowls called cirques into mountains.
Glaciers deposit till, which contains rock pieces of all different sizes.
Melting glacier ice also feeds streams that deposit outwash (because it is washed out of the glacier); the pieces in outwash are sorted by size (mostly sand in some places and mostly gravel in other places). Till and outwash often form ridges called moraines that outline the glacier.

Some of the features that glaciers erode and deposit are not produced in other ways, and such features from a time interval centered on roughly 20,000 years ago are observed now across broad areas of the Earth where glaciers do not now occur, suggesting that we have had an ice age or ice ages in the past.
This hypothesis of past ice ages has led to many predictions that were then confirmed, including that the land now should be rising where the ice was, and sinking just beyond where the ice was, and that the return of water to the oceans as the ice melted would have flooded old river valleys and killed shallow-water corals on the sides of islands.
The success of the ice-age hypothesis in predicting things that have since been observed, and the failure of other hypotheses to do so, give us high confidence that ice ages did occur.

The history of ice ages is clearer in ocean sediments than on land, especially in the isotopic composition of shells buried in mud on the sea floor.
Isotopically lighter water evaporates more easily.
An ice sheet is formed from water that evaporated, mainly from the ocean, and then snowed, so during an ice age the water remaining in the ocean is isotopically heavier than observed today, causing shells that grew then to be isotopically heavier.
The history of the isotopic composition of shells recovered from cores of mud from the ocean floor shows that the ice grew and shrank, with the biggest ice every 100,000 years, and smaller wiggles in ice size about 41,000 and 19,000 years apart.
These timings were predicted by the scientist Milankovitch decades before they were observed because they are the timings of features in Earth's orbit that control the distribution of sunshine on the planet.
Ice has grown globally when the far north was getting relatively little sunshine, especially in midsummer.
The rest of the world has cooled when ice grew in the north, even though parts of the world were getting extra sunshine, and the world has warmed when the ice melted in the north, even when some places were getting less sunshine.
This seemingly bizarre behavior occurred because the changing ice and other features of the climate changed atmospheric CO₂, which rose when the ice melted and fell when ice grew, and the CO₂ controlled global temperatures.

Ice sheets today cover about 10% of the land area; at the height of the ice age the ice sheets covered about 30% of modern land; central PA was just beyond the edge of ice that grew in Canada and flowed into the USA.
The high parts of Rocky Mountain National Park and the coastal parts of the NE Greenland National Parks are among the places that have permafrost today—soil at some depth is frozen year-round.
Rocks in permafrost regions are especially affected by freeze-thaw processes that break rocks, and by downhill soil creep (during the summer, the meltwater can't drain downward through the frozen soil beneath, so the soil gets very soggy and creeps easily).
These characteristics of permafrost regions contribute to the formation of distinctive features, which are observed in places such as central Pennsylvania that were just south of the ice-age ice sheets but which are not forming there today.
This shows that central Pennsylvania, and other such regions just south of the ice-age ice sheets, were very cold during the ice age.