Glaciers form when snow accumulates over many years and compresses into ice. As glaciers move over land under their own weight, they erode the surface through processes like plucking and scouring. Glaciers carry eroded material and deposit it elsewhere through till, moraines, outwash, and eskers when they melt. Continental glaciers once covered over 28% of Earth during ice ages but now cover around 10%, while valley glaciers still exist in mountainous regions and provide evidence of past glacial erosion.

1. Section 2: Glaciers Chapter 8: ErosionalForces

2. I. How Glaciers Form and Move When snow does not melt it piles up. As it accumulates slowly, the increasing weight of the snow becomes great enough to compress the lower layers into ice. Eventually there can be enough pressure on the ice so that it becomes plastic like. The mass slowly begins to flow in a thick, plastic like lower layer, and ice slowly moves away from its source. A large mass of ice and snow moving on land under its own weight is a GLACIER.

3. II. Ice Eroding Rock Glaciers are agents of erosion. As glaciers pass over land, they erode it, changing features on the surface. Glaciers then carry eroded material along and deposit it somewhere else. Glacial erosion and deposition change large areas of Earth’s Surface.

4. II. Ice Eroding Rock Plucking When glacial ice melts, water flows into cracks in rocks. Water refreezes in the cracks, expands, and fractures the rock. Pieces of rock are lifted out by the ice as shown. Results in boulders, gravel, and sand being added to the sides and bottom of the glacier.

5. II. Ice Eroding Rock Transporting and Scouring As a glacier moves forward over land huge volumes of sediment and rock can be transported. Plucked rock fragments and sand at its base scour and scrape the soil and bedrock like sandpaper against wood, eroding the ground below even more. When bedrock is gouged deeply by rock fragments being dragged along, marks such as those in figure 10 (next slide) are left behind. These marks, called grooves, are deep, long, parallel scars on rocks. Shallower marks are called striations. Grooves and striations indicate the direction in which the glacier moved.

6. II. Ice Eroding Rock

7. III. Ice Depositing Sediment When glaciers begin to melt they are unable to carry much sediment. Sediment drops, or is deposited, on the land. When glaciers melt and shrink back, it is said to retreat. As it retreats, a jumble of boulders, sand, clay, and silt is left behind. This mixture of different sized sediments is called TILL. Till deposits can cover huge areas of land. Thousands of years ago, huge ice sheets in the Northern U.S. left enough behind to fill valley completely and make these areas appear flat. Some examples are: wheat farms running NW from Iowa to Montana, some farmland in parts of Ohio, Indiana, and Illinois, and the rocky pastures of New England.

8. III. Ice Depositing Sediment Moraine Deposits Till also is deposited at the end of a glacier when it is not moving forward. This type of deposit does not cover a wide area. Rocks and soil are moved to the end of the glacier, much like items on the belts at Way-Mart. Because of this, a big ridge of material piles up that looks like it has been pushed along by a bulldozer. Such a ridge is called a MORAINE. Moraines are also deposited along the sides of a glacier.

9. III. Ice Depositing Sediment Outwash Deposits When glacial ice starts to melt, the melt-water can deposit sediment that is different from till. Material deposited by the melt-water from a glacier is called outwash. Melt-water carries sediments and deposits them in layers. Heavier sediments drop first, so bigger pieces of rock are deposited closer to the glacier. Outwash from a glacier can also form into a fan shaped deposit when the stream of melt-water deposits sand and gravel in front of the glacier.

10. III. Ice Depositing Sediment Eskers When a glacier melts, a winding ridge of sand and gravel, called and esker, is left behind. The deposit forms in a melting glacier when melt-water forms a river within the ice.

11. IV. Continental Glaciers Continental Glaciers cover ten percent of Earth, mostly near the poles in Antarctica and Greenland. Continental glaciers are huge masses of ice and snow. Thicker than some mountain ranges. Continental Glaciers have pieces that break off as icebergs.

12. IV. Continental Glaciers Climate Changes In the past Continental Glaciers covered as much as 28 percent of Earth. Periods of widespread glaciation are known as Ice Ages. During the recent Ice Age the average temperature was 5 degrees less than it is today. (Celsius) The last major advance of ice reached its maximum 18,000 years ago. Currently ice sheets began to recede, or move back, by melting.

13. V. Valley Glaciers Valley Glaciers Occur even in today’s warmer global climate. In the high mountains where the average temperature is low enough to prevent snow from melting during the summer, valley glaciers grow and creep along.

14. V. Valley Glaciers Evidence of Valley Glaciers If you visit the mountains, you can tell whether valley glaciers ever existed there. You might look for striations, then search for evidence of plucking. Glacial plucking often occurs near the top of a mountain where a glacier is mainly in contact with the solid rock. Valley glaciers erode bowl-shaped basins through plucking, called CIRQUES, into the sides of mountains.

15. V. Valley Glaciers Evidence of Valley Glaciers Arete (ah RAYT) If two valley glaciers form side by side a long ridge called an arete forms between them. If Valley Glaciers erode a mountain from several directions, a sharpened peak called a HORN might form.

16. V. Valley Glaciers Evidence of Valley Glaciers Valleys that have been eroded by glaciers have a different shape from those eroded by streams. Stream-eroded valleys are normally V-Shaped. Glacially eroded valleys are U-Shaped because a glacier plucks and scrapes soil and rock from the sides as well as the bottom.

17. VI. Importance of Glaciers Glaciers have had a profound effect on Earth’s surface. They have eroded mountaintops and transformed valleys. The Great Lakes were gouged out by glacial ice. Vast areas of the continents have sediments that were deposited by great ice sheets. Glaciers leave behind sediments that are economically important. The sand and gravel deposits from glacial outwash and eskers are important resources for construction of roads and buildings.