Tuesday, December 28, 2010

Deposition by Ice


Glaciers produce may kinds of debris and drifts. As the ice progresses, a thick layer of fine clay builds up underneath. The pressure may be enough to keep the water liquid at the base even though the temperature is below 0 degree C. This lubricates the flow and leaves mounds of clay called drumlins. T the snout of the glaciers, where ice is melting, it deposits the rest of its load of sediment and rock as a terminal moraine that can block in a subsequent lake. The moraine in from of New Zealand’s Franz Josef glacier is 430 meter high.


 If an ice sheet retreats, as happened at the end of the last ice age, it leaves the country side coated with what is known as boulder clay or till- a completely unsorted rock mixture ranging from the finest clay to house sized boulders. Sometimes a block of ice is left behind in the clay and when that melts it leaves a deep pond or kettle hole. 


Melting can result in stratigraphical puzzles with, for example, big blocks of ancient rock sitting randomly on top of much younger material. These are known as erratic and their rock type often gives clues to the path taken by the ice. Erratic are often deposited in areas of different rock type. They perch precariously if they were dropped by rapidly melting ice. Ice in snow fields high in mountains compacts and begins to flow, leaving a corrie, cirque at the head of the valley. Back to back cirques leave jagged arêtes and pyramidal peaks.