Arch, UtahErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
There are different ways to classify landforms, but there are three general categories: landforms that are built (depositional), landforms that are carved (erosional), and landforms that are made by movements of the Earth's crust (tectonic). Here are the most common erosional landforms.
This arch, in Arches National Park in Utah, formed by erosion of solid rock. Water is the sculptor, even in deserts like the high Colorado Plateau.
Rainfall acts in two ways to erode rock into an arch. First, rainwater is a very mild acid, and it dissolves cement in rocks with a calcite cement between its mineral grains. A shaded area or a crack, where water lingers, tends to erode faster. Second, water expands as it freezes, so wherever water is trapped it exerts a powerful force upon freezing. It's a safe guess that this second force did most of the work on this arch. But in other parts of the world, particularly in limestone regions, dissolution creates arches.
Another kind of natural arch is a sea arch.02of 31
Arroyo, NevadaErosional Landform Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
Arroyos are stream channels with flat floors and steep walls of sediment, found all over the American West. They are dry most of the year, which qualifies them as a type of wash.03of 31
Badlands, WyomingErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
A badlands is where deep erosion of poorly consolidated rocks creates a landscape of steep slopes, sparse vegetation, and intricate stream networks.
Badlands is named for a part of South Dakota that the first explorers, who spoke French, called "mauvaises terres." This example is in Wyoming. The white and red layers represent volcanic ash beds and ancient soils or weathered alluvium, respectively.
Although such areas are truly obstacles to travel and settlement, badlands can be bonanzas for paleontologists and fossil hunters because of the natural exposures of fresh rock. They are also beautiful in a way no other landscape can be.
The high plains of North America have spectacular examples of badlands, including Badlands National Park in South Dakota. But they occur in many other places, such as the Santa Ynez Range in southern California.04of 31
Butte, UtahErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
Buttes are small tablelands or mesas with steep sides, created by erosion.
The incomparable landscape of the Four Corners region, in the desert Southwest of the United States, is dotted with mesas and with buttes, their smaller siblings. This photo shows mesas and hoodoos in the background with a butte on the right. It's easy to see that all three are part of an erosional continuum. This butte owes its sheer sides to the thick layer of homogeneous, resistant rock in its middle. The lower part is sloping rather than sheer because it consists of mixed sedimentary layers that include weaker rocks.
A rule of thumb might be that a steep-sided, isolated flat-topped hill is a mesa (from the Spanish word for table) unless it's too small to resemble a table, in which case it's a butte. A larger tableland may have buttes standing beyond its edges as outliers, left behind after erosion has carved away the intervening rock. These can be called buttes témoins or zeugenbergen, French and German terms mean"witness hillocks."05of 31
Canyon, WyomingErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
The Grand Canyon of the Yellowstone is one of the greatest sights in Yellowstone National Park. It's also a great example of a canyon.
Canyons don't form everywhere, only in places where a river is cutting downward much faster than the weathering rate of the rocks it cuts. That creates a deep valley with steep, rocky sides. Here, the Yellowstone River is strongly erosive because it carries a lot of water at a steep gradient down from the high, uplifted plateau around the huge Yellowstone caldera. As it cuts its way downward, the sides of the canyon fall into it and are carried away.06of 31
Chimney, CaliforniaErosional Landform Pictures. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)
A chimney is a tall block of bedrock standing on a wave-cut platform.
Chimneys are smaller than stacks, which have a shape more like a mesa (see a stack here with a sea arch in it). Chimneys are taller than skerries, which are low-standing rocks that can be covered in high water.
This chimney lies off Rodeo Beach, just north of San Francisco, and probably consists of greenstone (altered basalt) of the Franciscan Complex. It is more resistant than the graywacke around it, and wave erosion has carved it to stand alone. If it were on land, it would be called a knocker.07of 31
Cirque, CaliforniaErosional Landform Pictures. Photo courtesy Ron Schott of Flickr under Creative Commons license
A cirque ("serk") is a bowl-shaped rock valley on the side of a mountain, often with a glacier or permanent snowfield in it.
Cirques are created by glaciers, grinding an existing valley into a rounded shape with steep sides. This cirque was undoubtedly occupied by ice during all of the many ice ages of the last two million years, but at the moment it contains only a névé or permanent field of icy snow. Another cirque appears in this picture of Longs Peak in the Colorado Rockies. This cirque is in Yosemite National Park. Many cirques contain tarns, clear alpine ponds nestled in the hollow of the cirque.
Hanging valleys are commonly formed by cirques.08of 31
Cliff, New YorkErosional Landform Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
Cliffs are very steep, even overhanging rock faces formed by erosion. They overlap with escarpments, which are large tectonic cliffs.09of 31
Cuesta, ColoradoErosional Landform Pictures. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)
Cuestas are asymmetric ridges, steep on one side and gentle on the other, that form by the erosion of gently dipping rock beds.
Cuestas like these north of U.S. Route 40 near Dinosaur National Monument at the locality of Massadona, Colorado, emerge as harder rock layers have their softer surroundings eroded away. They are part of a larger structure, an anticline that is plunging toward the right. The sets of cuestas in the center and right are dissected by stream valleys, whereas the one on the left edge is undivided. It is better described as an escarpment.
Where rocks are tilted steeply, the erosional ridge they make has roughly the same slope on both sides. That type of landform is called a hogback.10of 31
Gorge, TexasErosional Landform Pictures. Photo courtesy Southwest Research Institute
A gorge is a ravine with nearly vertical walls. This gorge was cut when heavy rains pushed a flood over Canyon Lake Dam in central Texas in 2002.11of 31
Gulch, CaliforniaErosional Landform Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
A gulch is a deep ravine with steep sides, carved by flash floods or other torrential streamflows. This gulch is near Cajon Pass in southern California.12of 31
Gully, CaliforniaErosional Landform Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
A gully is the first sign of serious erosion of loose soil by running water, although it does not have a permanent stream in it.
A gully is part of a spectrum of landforms created by running water erodes sediment. Erosion starts with sheet erosion until running water concentrates into small irregular channels called rills. The next step is a gully, like this example from near the Temblor Range. As a gully grows, the stream course would be called a gulch or ravine, or maybe an arroyo depending on various features. Usually, none of these involve erosion of bedrock.
A rill can be ignored -- an offroad vehicle can cross it, or a plow can wipe it out. A gully, though, is a nuisance to everyone except the geologist, who can get a clear look at the sediments exposed in its banks.13of 31
Hanging Valley, AlaskaErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
A hanging valley is one with an abrupt change in elevation at its outlet.
This hanging valley opens onto Tarr Inlet, Alaska, part of Glacier Bay National Park. There are two main ways of creating a hanging valley. In the first, a glacier excavates a deep valley faster than a tributary glacier can keep up. When the glaciers melt, the smaller valley is left suspended. Yosemite Valley is well known for these. The second way a hanging valley form is when the sea erodes the coast faster than a stream valley can cut down to grade. In both cases, the hanging valley commonly ends with a waterfall.
This hanging valley is also a cirque.14of 31
Hogbacks, ColoradoErosional Landform Pictures. Photo (c) 2007 Andrew Alden, licensed to About.com (fair use policy)
Hogbacks form when steeply tilted rock beds are eroded. The harder rock layers slowly emerge as hogbacks like these south of Golden, Colorado.
In this view of the hogbacks, the harder rocks are on the far side and the softer rocks that they protect from erosion are on the near side.
Hogbacks get their name because they resemble the high, knobby spines of pigs. Usually, the term is used when the ridge has roughly the same slope on both sides, which means that the resistant rock layers are steeply tilted. When the resistant layer is tilted more gently, the softer side is steep while the hard side is gentle. That type of landform is called a cuesta.15of 31
Hoodoo, New MexicoErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
Hoodoos are tall, isolated rock formations that are common in dry regions of sedimentary rock.
In a place like central New Mexico, where this mushroom-shaped hoodoo stands, erosion commonly leaves bits of resistant rock protecting the weaker rock layer beneath it.
The big geologic dictionary says that only a tall formation should be called a hoodoo; any other shape -- a camel, say -- is called a hoodoo rock.16of 31
Hoodoo Rock, UtahErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
Hoodoo rocks are grotesquely shaped rocks, like hoodoos, except that they aren't tall and thin.
Deserts create many strange-looking landforms from the rocks beneath them, like arches and domes and yardangs and mesas. But a particularly grotesque one is called a hoodoo rock. Dry-climate erosion, without the softening effects of soil or humidity, brings out the details of the sedimentary joints and cross bedding, carving suitable formations into suggestive shapes.
This hoodoo rock from Utah shows cross-bedding pretty clearly. The lower part is made of sandstone beds dipping one direction, while the middle part dips in another. And the top part consists of contorted strata that got that way from some sort of underwater landslide while the sand was being laid down, millions of years ago.17of 31
Inselberg, CaliforniaErosional Landform Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
Inselberg is German for "island mountain." An inselberg is a knob of resistant rock in a wide erosional plain, typically found in deserts.18of 31
Mesa, UtahErosional Landform Pictures. Photo (c) 1979 Andrew Alden, licensed to About.com (fair use policy)
Mesas are mountains with flat, level tops, and steep sides.
Mesa is Spanish for table, and another name for mesas is table mountains. Mesas form in arid climates in regions where nearly flat rocks, either sedimentary beds or large lava flows, serve as caprocks. These resistant layers protect the rock beneath them from eroding.
This mesa overlooks the Colorado River in northern Utah, where a strip of lush farmland follows the stream between its steep rock walls.19of 31
Monadnock, New HampshireErosional Landform Pictures. Photo courtesy Brian Herzog of Flickr under Creative Commons license
Monadnocks are mountains left standing in low plains that eroded around them. Mount Monadnock, eponym of this landform, is hard to photograph from the ground.20of 31
Mountain, CaliforniaErosional Landform Pictures. Photo courtesy Craig Adkins, all rights reserved
Mountains are landforms at least 300 meters (1,000 feet) high with steep and rocky sides and a small top, or summit.
Cave Mountain, in the Mojave Desert, is a good example of an erosional mountain. The 300-meter rule is a convention; sometimes people limit mountains to 600 meters. Another criterion sometimes applied is that a mountain is something worthy of being given a name.
Volcanoes are also mountains, but they form by deposition.
Visit the Gallery of Peaks21of 31
Ravine, FinlandErosional Landform Pictures. Photo courtesy daneen_vol of Flickr under Creative Commons license
Ravines are small, narrow depressions carved by running water, between gullies and canyons in size. Other names for them are cloves and cloughs.22of 31
Sea Arch, CaliforniaErosional Landform Pictures. Photo (c) 2003 Andrew Alden, licensed to About.com (fair use policy)
Sea arches form by wave erosion of coastal headlands. Sea arches are very temporary landforms, in both geologic and human terms.
This sea arch at Goat Rock Beach south of Jenner, California, is unusual in that it sits offshore. The usual method of forming a sea arch is that a headland focuses incoming waves around its point and onto its flanks. The waves erode sea caves into the headland that eventually meet in the middle. Soon enough, maybe in a few centuries at most, the sea arch collapses and we have a sea stack or a tombolo, like the one just north of this spot. Other natural arches form inland by much gentler means.23of 31
Sinkhole, OmanErosional Landform Pictures. Photo courtesy Trubble of Flickr under Creative Commons license
Sinkholes are closed depressions that arise in two events: groundwater dissolves limestone, then the overburden falls into the gap. They are typical of karst. The more general term for karstic depressions is doline.24of 31
StrathErosional Landform Pictures. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)
Straths are bedrock platforms, former stream valley floors, that have been abandoned as the stream that cut them formed a new stream valley at a lower level. They may also be called stream-cut terraces or platforms. Consider them the inland version of wave-cut platforms.25of 31
Tor, CaliforniaErosional Landform Pictures. Photo (c) 2003 Andrew Alden, licensed to About.com (fair use policy)
A tor is a particular kind of hill -- bare rock, sticking high above its surroundings, and often displaying rounded and picturesque shapes.
The classic tor occurs in the British Isles, granite knobs rising from the gray-green moors. But this example is one of many in California's Joshua Tree National Park and elsewhere in the Mojave Desert where granitic rocks exist.
The rounded rock forms are due to chemical weathering under the thick soil. Acid groundwater penetrates along jointing planes and softens the granite into a loose gravel called grus. When climate changes, the soil mantle is stripped away to reveal the bones of the bedrock beneath. The Mojave was once much wetter than today, but as it dried out this distinctive granite landscape emerged. Periglacial processes, related to the frozen ground during the ice ages, may have helped remove the overburden of the tors of Britain.
For more pictures like this, see the Joshua Tree National Park Photo Tour.26of 31
Valley, CaliforniaErosional Landform Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
A valley is any piece of low ground with the high ground around it.
"Valley" is a very general term that implies nothing about the shape, character or origin of the landform. But if you asked most people to draw a valley, you would get a long, narrow notch between ranges of hills or mountains with a river running in it. But this swale, which runs along the trace of the Calaveras fault in central California, is also a perfectly good valley. The types of valleys include ravines, gorges, arroyos or wadis, canyons, and more.27of 31
Volcanic Neck, CaliforniaErosional Landform Pictures. Photo (c) 2003 Andrew Alden, licensed to About.com (fair use policy)
Volcanic necks emerge as erosion strips away the ash and lava mantle of volcanoes to reveal their hard magma cores.
Bishop Peak is one of the nine Morros. The Morros is a string of long-extinct volcanoes near San Luis Obispo, in central coastal California, whose magma cores have been exposed by erosion in the 20 million years since they last erupted. The hard rhyolite inside these volcanoes is much more resistant than the soft serpentinite -- altered seafloor basalt -- that surrounds them. This difference in rock hardness is what lies behind the appearance of volcanic necks. Other examples include Ship Rock and Ragged Top Mountain, both listed among the peaks of the Mountain Western states.28of 31
Wash or Wadi, Saudi ArabiaErosional Landform Pictures. Photo courtesy Abdullah bin Saeed, all rights reserved
In America, a wash is a stream course that has water only seasonally. In southwest Asia and north Africa, it is called a wadi. In Pakistan and India, it is called a nullah. Unlike arroyos, washes may be any shape from flat to rugged.29of 31
Water Gap, CaliforniaErosional Landform Pictures. Photo (c) 2003 Andrew Alden, licensed to About.com (fair use policy)
Water gaps are steep-sided river valleys that appear to have cut through a range of mountains.
This water gap is in the hills on the west side of California's Central Valley, and the gorge was created by Corral Hollow Creek. In front of the water, a gap is a large, imperceptibly sloping alluvial fan.
Water gaps can be created in two ways. This water gap was made the first way: the stream was there before the hills began to rise, and it maintained its course, cutting down as fast as the land rose. Geologists call such a stream an antecedent stream. See three more examples: Del Puerto and Berryessa gaps in California and Wallula Gap in Washington.
The other way of forming a water gap is through stream erosion that uncovers an older structure, such as an anticline; in effect, the stream is draped over the emerging structure and cuts a gorge across it. Geologists call such a stream a resequent stream. Many water gaps in the eastern U.S. mountains are of this type, as is the cut made by the Green River across the Uinta Mountains in Utah.30of 31
Wave-Cut Platform, CaliforniaErosional Landform Pictures. Photo (c) 2008 Andrew Alden, licensed to About.com (fair use policy)
The flat surface on this northern California headland is a wave-cut platform (or marine terrace) that now lies above the sea. Another wave-cut platform lies under the surf.
The Pacific shore in this photo is a place of wave erosion. The surf chews at the cliffs and washes their pieces offshore in the form of sand and pebbles. Slowly the sea eats into the land, but its erosion cannot extend in the downward direction beyond the base of the surf zone. Thus the waves carve out a fairly level surface offshore, the wave-cut platform, divided into two zones: the wave-cut bench at the foot of the wave-cut cliff and the abrasion platform farther from shore. The bedrock knobs that survive on the platform are called chimneys.31of 31
Yardang, EgyptErosional Landform Pictures. Photo courtesy Michael Welland, all rights reserved
Yardangs are low ridges carved in soft rock by persistent winds in flat deserts.
This field of yardangs formed in poorly lithified sediments of a former lake bed in Egypt's the Western Desert. Steady winds blew away the dust and silt, and in the process, the windblown particles carved these remnants into the classic form called "mud lions." It is an easy speculation that these silent, evocative shapes inspired the ancient motif of the sphinx.
The higher "head" end of these yardangs faces into the wind. The front faces are undercut because wind-driven sand stays near the ground, and erosion is concentrated there. Yardangs may reach 6 meters in height, and in some places, they have rugged tops held up by smooth, narrow necks sculpted by thousands of sandstorms. They may also be low ridges of rock without picturesque protuberances. An equally important part of a yardang is the pair of wind-blown excavations, or yardang troughs, on either side of it.