14Step 4: The ice begins to move. Once the ice becomes 50m thick, the pressure on the bottom of the glacier is so great that it becomes plastic-like and flows like silly putty out from its thickest point.
15How do glaciers erode the surface? Plucking – as a glacier flows over the ground, it loosens and lifts blocks of rock into the bottom of the glacier.As the glacier moves, these rocks scrape deep grooves, called striations, into the rock below.
16Glaciers pick up lots of sediment as they advance over the land.
17Melting GlaciersAs glaciers move over the land (advance), they carry sediment with them. As they melt (retreat), the sediment that they were carrying is dropped into unsorted piles called moraines.
18How do glaciers leave their mark on the earth’s surface as they retreat? Melting glaciers deposit moraine (piles of unsorted sediment that they were carrying) all along the front edge of the glacier
19Why do scientists believe that glaciers once covered Michigan?
20Moraine Deposits = unsorted sediments Moraines are made of unsorted sediments. Only mass movements and glaciers deposit unsorted sediments.Since there are no large hills or mountains in Michigan for this sediment to fall down, it must have been deposited by the glaciers.
21Moraine Deposits have the same shape as the Great Lakes. Michigan moraines run parallel to the shoreline.The same process that formed the moraines formed the Great Lakes.
22Each of the Great Lakes began as a river. Image from Earth Science, Tarbuck and Lutgens, 2003
23When the climate cooled… Ice advanced over the land, moving southward from Canada over the Great Lakes Region.
24As the climate cooled… The rivers froze. Glaciers moved through them – widening and deepening them to form today’s lake bottoms.
25The sediment removed from the river valleys was deposited in the moraines covering our state. This is why the moraines run parallel to the shorelines of the Great Lakes.
26When the climate began to warm, the glaciers began to melt and retreat.
27The fresh water from the melting glaciers filled in the deep U-shaped valleys that they had carved and turned them into the lakes we have today.
28What other evidence do we have that glaciers once covered our state? Depositional features such as drumlins and kettle lakes.Kalkaska, Michigan
29Kettle LakesKettle lakes form when blocks of ice break off the front edge of a glacier, become buried by sediment. The ice melts leaving a hole which fills with water creating a lake.
31DrumlinsHills of sediment deposited by the glacier
32Why do scientists believe that glaciers once covered Michigan? Michigan is covered with unsorted moraine deposits.The moraine deposits follow the outline of Great Lakes.Other depositional features such as drumlins and kettle lakes are found throughout Michigan.