How do glaciers affect land?
Glacier National Park, in Montana, U.S.A., also shows a variety of glaciated valleys. Here, part of the mountain has been cut away by a glacier.
—U.S. Air Force collection/NSIDC
Glaciers not only transport material as they move, but they also sculpt and carve away the land beneath them. A glacier's weight, combined with its gradual movement, can drastically reshape the landscape. Over hundreds or even thousands of years, the ice totally changes the landscape. The ice erodes the land surface and carries the broken rocks and soil debris far from their original places, resulting in some interesting glacial landforms.
Common all over the world, glaciated valleys are probably the most readily visible glacial landform. Similar to fjords, they are trough-shaped, often with steep vertical cliffs where entire mountainsides were removed by glacial action. One of the most striking examples of glaciated valleys can be seen in Yosemite National Park, where glaciers literally sheared away mountainsides, creating deep valleys with vertical walls.
Fjords, such as those in Norway, are long, narrow coastal valleys that were originally carved out by glaciers. Steep sides and rounded bottoms give them a trough-like appearance. Because of glacial erosion on the below sea level land surface, when glaciers finally disappear, sea water covers the valley floor.
The Matterhorn in Switzerland was carved away by glacial erosion. The cirque on this side still contains a glacier, and to the far left, a hanging glacier clings precariously to the side of the peak. Notice the crevasses on each of the glaciers. This photograph dates from 1894.
The famous Matterhorn in Switzerland displays three types of glacial erosion:
- Cirques are created when glaciers erode backwards, into the mountainside, creating rounded hollows shaped like a shallow bowls.
- Aretes are jagged, narrow ridges created where the back walls of two cirque glaciers meet, eroding the ridge on both sides.
- Horns, such as the famous Matterhorn in Switzerland, are created when several cirque glaciers erode a mountain until all that is left is a steep, pointed peak with sharp, ridge-like aretes leading up to the top.
Fjords, glaciated valleys, and horns are all erosional types of landforms, created when a glacier cuts away at the landscape. Another type of glacial landform is created by deposition, or what a glacier leaves as it retreats or melts away.
Till is material that is deposited as glaciers retreat, leaving behind mounds of gravel, small rocks, sand and mud. It is made from the rock and soil ground up beneath the glacier as it moves. Glacial till can form excellent soil for farmland.
An eroded moraine juts above the landscape in the Rhone Valley, Switzerland. Giant boulders stud some of the pinnacles, left behind by a retreating glacier. A small tunnel has been cut through the moraine, seen at the center of the photograph.
—H.F. Reid Collection, 1902/NSIDC
Material a glacier picks up or pushes as it moves forms moraines along the surface and sides of the glacier. As a glacier retreats, the ice literally melts away from underneath the moraines, so they leave long, narrow ridges that show where the glacier used to be. Glaciers don't always leave moraines behind, because sometimes the glacier's own meltwater carries the material away.
Streams flowing from glaciers often carry some of the rock and soil debris out with them. These streams deposit the debris as they flow. Consequently, after many years, small steep-sided mounds of soil and gravel begin to form adjacent to the glacier, called kames.
Kettle lakes form when a piece of glacier ice breaks off and becomes buried by glacial till or moraine deposits. Over time the ice melts, leaving a small depression in the land, filled with water. Kettle lakes are usually very small, and are more like ponds than lakes.
Meltwater flows out from under the terminus of Forno Glacier in this photograph from 1899.
—H. F. Reid Collection, 1899/NSIDC
Glaciers leave behind anything they pick up along the way, and sometimes this includes huge rocks. Called erratic boulders, these rocks might seem a little out of place, which is true, because glaciers have literally moved them far away from their source before melting away.
Drumlins are long, tear-drop-shaped sedimentary formations. What caused drumlins to form is poorly understood, but scientists believe that they were created subglacially as the ice sheets moved across the landscape during the various ice ages. Theories suggest that drumlins might have been formed as glaciers scraped up sediment from the underlying ground surface, or from erosion or deposition of sediment by glacial meltwater, or some combination of these processes. Because the till, sand and gravel that form drumlins, are deposited and shaped by glacier movement, all drumlins created by a particular glacier face the same direction, running parallel to the glacier's flow. Often, hundreds to thousand of drumlins are found in one place, looking very much like whalebacks when seen from above.