Erosion – Background

The landscape around us is a patchwork of mountains and valleys, steep cliffs and gentle slopes, lakes, ponds, and puddles, stitched together with a network of rivers and streams. The features of our landscape tell a story about the process of erosion, in which landforms are worn away and their fragments carried off to another place. Jagged mountains became rolling hills, flat plains became deep river gorges, steep bluffs crumble into the sea.

Erosion is the process by which rocks and sediments are loosened and moved to different places on the Earth’s surface, where they may be deposited and become parts of other landforms, or buried to eventually become solid rock again. The agents of erosion – wind, water, ice, and gravity – are powerful forces constantly at work shaping and reshaping the land.

Everything moves downhill, from boulders to pebbles to soil. This is largely due to the force of gravity, pulling everything towards the center of the Earth. The movement of loose material downhill, called mass wasting, can be a sudden event like a landslide or mudslide, or it can be a gradual process, so slow as to be nearly imperceptible. Soil creep is the gradual downhill movement of soil on a slope. It’s what causes trees, fence posts, and utility poles at the base of hillsides to gradually take on a downhill lean. How cohesive a material is, and the steepness of a slope, are factors affecting whether and when it will slide. Loose materials like rock fragments, sand, and gravel tend to settle at a particular angle and remain at rest unless disturbed. Each kind has an angle of repose, the steepest slope at which a material is stable. Normally about thirty degrees for dry sand, it varies with size and shape of the grains and amount of moisture. However, heavy rain, vibrations from earthquakes, even freezing and thawing, can upset the balance. Landscapers pay close attention to the safe angles for different materials when doing excavation work.

Much of our landscape today was sculpted by the glaciers that covered the land with a mile-thick sheet of ice during the last ice age, and receded around ten thousand years ago. Glaciers are huge masses of flowing ice that build up in places where more snow accumulates every year than melts away. They move only a few centimeters a year, but they cause big changes in the landscape. Glaciers erode in two main ways. At the base of the glacier, loose gravel and rocks become frozen into the ice and are carried along, scouring and abrading the surfaces over which they move. Glaciers also pluck rocks from the landscape and carry them along as well. Mounds of rock debris, called glacial till, pile up along the sides, middle, and terminal end of a glacier, and they are left behind when the glacier retreats. The action of glaciers leaves many signs – striations in rocks or polished surfaces, U-shaped valleys (compared to the steep, V-shaped valleys carved by rivers), rounded mountaintops, scooped-out basins that fill with water to become lakes and ponds. Glacial erratics, large boulders carried far from their source and left behind, seem like cairns marking a glacier’s pathway.

Running water is an extremely important agent of erosion, affecting every continent on Earth. Water from raindrops begins the process of erosion, the force of each drop displacing a bit of loose soil and sand. Small grains may be splashed outwards or picked up and carried along as the water moves downhill. Rain on bare soil creates gullies, which become rivulets, eventually joining other rivulets and feeding into creeks and streams. Rainfall moves large amounts of sediment, especially from bare, exposed soil. Seed blankets and hydro-seeding are used to protect exposed soil from being washed away and to promote plant growth, for roots hold soil in place. When running over impervious surfaces like building roofs and parking lots, rain picks up sand, silt, and pollutants and carries them into nearby streams and lakes. Rain gardens, living roofs, and permeable pavement are modern methods of erosion control that help to slow the flow of water, filter out pollutants, and trap sediments.

Rivers have an immense power to erode the land. Some of the most dramatic features of our landscape, like the Grand Canyon or Niagara Falls, were made by stream erosion. Rivers erode by removing soil and rocks from stream banks, cutting downwards through the stream bed, and wearing away the land around the headwaters. The force of the water can undercut banks of soft mud or sand. In addition, the stream carries suspended sand and silt and sometimes even larger rocks, which help abrade the stream banks and scour the stream bed. Pebbles and cobbles carried by stream waters also knock into each other, becoming smoother and rounder the farther they travel. In places where a river gets wider or less steep, the water slows down and is no longer able to carry the same load. The sediments settle out in order of size, from large to small. Rivers are good at sorting sediments by size. Larger, jagged fragments generally settle out farther upstream, and the finest, smoothest particles are deposited near the river’s mouth.

Erosion of stream banks is a natural process, and rivers have a dynamic, changing nature. But sometimes the things we do – deepening channels, armoring stream banks, straightening bends – can cause an increase in erosion elsewhere. A stream is a unified system, and change in one part will affect other parts as well. One effective way to keep streams from eroding is to keep a forested buffer along the banks. Trees, much more than grasses, hold soil and rocks in place. In addition, they provide a corridor for animals and needed shade for stream inhabitants like fish and aquatic insects. Many banks have been restored to a forested condition by planting young native trees and shrubs, which give reforestation a head start.

Some of the most dramatic erosion can be seen along coastlines, where ocean waves eat away at dunes and rocky cliffs by battering them with sand and gravel and carrying the loose material away. Much less obvious, because it is out of view, is erosion underground. Groundwater often contains dilute carbonic acid, formed from carbon dioxide in the air and soil. The acid reacts with and dissolves minerals in rocks like limestone and gypsum, forming caves and sinkholes. Groundwater carries the minerals away to be deposited elsewhere, sometimes as beautiful formations like stalactites and stalagmites. The chemical or mechanical disintegration of rocks above ground, called weathering, is caused by freezing and thawing, by plants, or by chemicals in the air or in rain. Weathering creates sediments, the mineral grains that form a part of the soil so essential to terrestrial plants.

Wind is also an agent of erosion, sweeping away loose sand, silt, and dust and carrying it off to be deposited in new places. Wind abrasion is not thought to be a significant agent of erosion, though in deserts, rocks sculpted by sandblasting can be found. But wind is very important in picking up huge amounts of loose sediment and moving it great distances. In general, wind moves only small particles – sand, silt, and clay – leaving behind coarser material that it cannot lift. Wind erosion is most important in deserts and dry areas with exposed soil. Sand dunes in deserts and along coastlines are formed and sculpted by the wind. A dune begins to form when an obstacle like a rock creates a wind shadow, an area of stiller air behind it, where sand settles out and piles up. Deposits of wind-blown dust, called loess, provide some of the most fertile soils in the world, including those in much of our Midwestern prairies and plains.

Erosion is only one part of the story of our changing landscape, for tectonic forces inside the Earth are constantly building new mountains, even as erosion is wearing them down. Erosion is both a destructive force and a constructive one. It scours out lakes, cuts river channels, and breaks rocks into fragments, and in so doing, it creates a diversity of habitats for living things and the soil on which our food supply depends.

Suggested Reading

Hamblin, W. Kenneth and Eric H. Christiansen. Earth’s Dynamic Systems. 10th ed. Upper Saddle River, NJ: Prentice-Hall, 2004.

Plummer, Charles C., Diane H. Carlson, David McGeary. Physical Geology. 11th ed. Boston, MA: McGraw-Hill, 2007.

Tarbuck, Edward J., Frederick K. Lutgens, Denis G. Tasa. Earth: An Introduction to Physical Geology. 11th ed. Upper Saddle River, NJ: Prentice-Hall, 2013.

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