GLACIATION
The present extent of glacier ice masses represents 11% of the earth’s total land area.
Today, most of this ice is contained in the ice sheets which cover Antarctica and Greenland.
At the time of maximum glaciation, ice masses extended over large areas of the Northern hemisphere. Few parts of the earth were unaffected by ice action.
The period during which world-wide glaciation occurred was the PLEISTOCENE Ice Age. It lasted for 2 million years up to the end of the last glaciation about 12,000 years BP (before the present)
During the Pleistocene, considerable variations took place in the extent of ice cover. Major periods of glaciation (ice advance) were separated by periods of temperate climate known as interglacials, and individual glacials were interrupted by shorter periods of milder conditions called interstadials (ice retreat)
Penck and Bruckner produced a chronology of the 4 major ice advances in the Alps: Gunz, Mindel, Riss and Wurm.
During the Pleistocene, ice covered as much as one third of the earth’s land area, the volume of ice equivalent to a 200m drop in sea level.
The largest ice mass covered much of N. America, spreading out from Labrador, Hudson Bay and the western mountain ranges. Eurasia was influenced by several smaller ice sheets, the largest of which developed over Scandinavia.
Outside of Antarctica, relatively little of the Southern hemisphere was affected by ice.
The regions beyond the margins of the main glacial areas came under the influence of periglaciation.
NIVATION
Also called snowpatch erosion: forms rounded hollows and valley heads. Snow accumulates in hollows. Melting snow (on diurnal or annual cycle), provides water which can soak into the ground. This freezes at nights, and over time, the rock breaks up. The resultant debris is then removed by a process of transport called solifluction.
The result is a rounded hollow called a nivation cirque, as it comes to resemble the glacial feature. Frost-shattered debris from the headwall can slide over the sloping snow surface and collect as a ‘protalus rampart’ at the front of the hollow.