Over the last 2.5 million years, during the geological epoch known as the Pleistocene, no other force on Earth has shaped and reshaped the landscape as much as ice. During this period, and up to the end of the last ice age about 12,000 years ago, the Earth has undergone numerous periods of continental-scale glaciation. Each time the ice sheets advanced and retreated they sculpted the landscape on a massive scale.
Even today, large ice sheets cover Greenland and Antarctica and hundreds of smaller mountain or alpine glaciers are found at high elevation all over the world. Though most of these glaciers are currently in retreat due to a warming climate, they continue to erode and shape the landscape. In this article we consider the major landforms and landscape features produced by the erosional and depositional action of glaciers, both today and in the past.
Glaciers often act like huge conveyor belts, with ice flowing from an area of accumulation (where more ice forms from snowfall than is lost from melting, evaporation, etc.) to a zone of ablation (where more ice is lost than accumulates). For alpine glaciers, this means the ice flows downhill through a valley, and for ice sheets, ice flows from the center of the sheet toward the edges. As the ice moves, it frequently picks up an drags along rocks, soil, and other debris (collectively called glacial till). Regardless of whether the glacier is advancing or retreating, the ice will continue to move this material toward the edge, where it is deposited into mounds called moraines.
If the glacier forms a pile of till where it has advanced the furthest, this is called a terminal moraine. Often glaciers will not retreat quickly from this most advanced position, and will therefore build up a significant moraine at its terminus. As the glacier retreats it may also pause from time to time, when accumulation again equals ablation. Each time the glacier stops for a while, it is able to construct additional mounds of till known as recessional moraines. Thus, in a post-glacial landscape it isn’t unusual to have one terminal moraine and several recessional moraines.
Moraines can also develop along the sides of glaciers as material falls downhill from higher elevations and accumulates along the glaciers’ margins. These lateral moraines, as they are called, are commonly found among alpine glaciers. If two or more valley glaciers (alpine glaciers flowing downhill and through a valley) join, lateral moraines can get caught in between the ice flows forming what is then called a medial moraine. Large valley glaciers formed from the merger of multiple smaller ice flows may look stripped due the presence of medial moraines.
Ridges and Valleys
As they slowly migrate downhill under the influence of gravity, alpine glaciers tend to carve out steep valleys and ridges. The constant grinding of glacial ice and snow against either side of a mountain ridge causes the ridge to become steeper and sharper over time, forming what is known as an arête. Material is also removed from the valley itself, creating wider and wider U-shaped valleys. At the end of the last ice age, some of these U-shaped valleys along the coast became inundated with water, forming what we know today as fjords. There are, however, many dry U-shaped glacial valleys, including the spectacular Yosemite Valley in California.
Many alpine glaciers originate in bowl-shaped depressions high in elevation at the head of mountain valleys. These depressions, called cirques, can give rise to cirque glaciers (those actually within the cirque) and valley glaciers that move downhill between mountain ridges. Where three or more cirques come together, a very steep mountain peak may form, known as a horn. The Matterhorn in the Alps is a well-known example.
When glaciers retreat and begin to melt, they often leave behind depressions in the ground that fill with glacial meltwater. These [mostly shallow] depressions in the ground are called kettles, and when they fill with water they are known as kettle lakes. Kettle lakes often fill with sediment and are usually not very deep; however, as is the case in Minnesota, they can occur in large numbers in the post-glacial landscape.
Some glacial lakes can be much larger in size than the average kettle lake. The Great Lakes in North America are the largest glacial lakes in the world, and formed around the end of the last major ice age (~12,000 BCE) when the Laurentide ice sheet left massive depressions in what is today the Great Lakes basin. As the ice sheet retreated, meltwater filled the depressions carved by ice and continued to be feed by groundwater. Other lakes have formed by huge waterfalls draining off the sides of continental glaciers, scouring away the land underneath.
Meltwater has also accumulated on top of and alongside continental ice sheets, forming lakes such as the former Lake Missoula in present day western Montana. The lake was massive; it covered 3,000 square kilometers and contained about half the volume of Lake Michigan. The water was held back by an enormous ice dam, some 600 meters tall, than was breached several times, causing massive floods across the northwest U.S. A couple hundred cubic kilometers of sediment were scoured out by these floods, forming broad canyons.
Glaciers are often quite ‘dirty’, carrying with them tons of rock, sediment, and other debris. Much of this material is simply dropped by the glacier itself (such as the case with moraines), but some is transported away by glacial meltwater coming off the top of the glacier, and from underneath. Meltwater that forms on the surface of a glacier can flow down to the base by entering deep well-like holes in the ice known as moulins. This water may be channeled along the underside of the glacier through tunnels, which then empty out along the edges of the glacier. Meltwater can carry a significant amount of sediment, which then gets deposited either underneath the glacier or out into the adjacent landscape as an outwash plain. The outwash plain, also called a sandur, consists of a layer of mixed sediment and debris deposited both directly by the glacier and by glacial melwater.
Occasionally, the channels of water flowing underneath glaciers become clogged with sediment, much like old pipes in your home can become clogged with minerals and other materials. Once the glacier retreats, a narrow, snake-like hill known as an esker is left behind. Eskers rise above the landscape like inverted stream channels, many even appearing to meander back and forth like a river. Mound-like features known as kames may also form as sediment gets trapped and piles up underneath a retreating glacier.