Glaciers

Ice ages

 Ice Ages are periods lasting millions of years when the Earth is so cold that polar ice caps grow huge. There are various theories about why they occur . There have been four Ice Ages in the last 1000  years.

 

Ice-land

 The most recent Ice Age - called the Pleistocene Ice Age  began about 2 million years ago.In an Ice Age the weather varies between cold spells called glacials and warm spells called interglacials. There were 17 glacials and interglacials in the last 1.6 million years of the Pleistocene Ice .

The people of the Ice Age risked their lives to hunt the fierce woolly mammoth. It was a good source of meat, skins, bones and ivory. California may have looked something like this 18,000 years ago when it was on the fringes of an ice sheet. 
 Where Washington and London are today, the ice was 1.5 km thick 18,000 years ago. Glaciers are rivers of slow- moving ice. They form in mountain regions when it is too cold for snow to melt.They flow down through valleys, creeping lower until they melt in the warm air lower down.Glaciers form when new snow falls on old. The new snow compacts the old snow into denser snow called firn. The dense ice in glaciers is made from thousands of years of snow. As new snow fell, the old snow beneath it became squeezed more and more in a process called firnification,squeezed out so it looks like ice. 
As more snow falls, firn gets more compacted and becomes glacier ice flowing slowly downhill. Where glaciours forms Now a days glaciers form only in high mountains and towards the North and South Poles. In the Ice Ages glaciers were widespread and left glaciated landscapes in many places that are now free of ice. crevasse As glaciers move downhill, they bend and stretch, opening up deep cracks called crevasses. Sometimes these occur where the glacier passes over a ridge. 
The biggest crevasse is often called the bergschrund. It forms when the ice pulls away from the back wall of the hollow where the glacier starts. Where the underside of a glacier is warmish (about 0°C), it moves by gliding on a film of water, made as pressure melts the glacier’s base. This is basal slip. Where the underside of a glacier is coldish (below 0°C), it moves as if layers are slipping over each other like a pack of cards. 
This is internal deformation. Valley glaciers are glaciers that flow in existing valleys. Cirque glaciers are small glaciers that flow from hollows high up. Alpine valley glaciers form when several cirque glaciers merge. Piedmont glaciers form where valley glaciers join as they emerge from the mountains. Glaciers move slowly but their sheer weight and size give them enormous power to shape the landscape. 
Over tens of thousands of years glaciers carve out winding valleys into huge, straight U-shaped troughs. Glaciers may slice off tributary valleys to leave them ‘hanging’, with a cliff edge high above the main valley. Hill spurs (ends of hills) may also be truncated. Cirques, or corries, are armchair-shaped hollows carved out where a glacier begins high up in the mountains. 
 After an Ice Age, glaciers leave behind a dramatically altered landscape of deep valleys and piles of debris. Aretes are knife-edge ridges that are left between several cirques as the glaciers in them cut backwards. Drift is a blanket of debris deposited by glaciers. Glaciofluvial drift is left by the water made as the ice melts. Till is left by the ice itself. 
Drumlins are egg-shaped mounds of till. Eskers are snaking ridges of drift left by streams under the ice. Moraine is piles of debris left by glaciers. Proglacial lakes are lakes of glacial meltwater dammed up by moraine.
 Periglacial conditions are found on the tundra of northern Canada and Siberia and on nunataks, which are the hills that protrude above ice sheets and glaciers. aping the land 0 When frozen soil melts it becomes so fluid that it can creep easily down slopes, creating large tongues and terraces. Frost heave is the process when frost pushes stones to the surface as the ground freezes.
 After frost heave, large stones roll down leaving the fine stones on top. This creates intricate patterns on the ground.On flat ground, quilt-like patterns are called stone polygons. On slopes, they stretch into stone stripes. .