A landform is a natural feature formed by the erosion or accumulation of soil or rock on Earth’s surface. Most landforms are produced by the actions of weathering and erosion, carving away material from higher elevations and depositing it at lower elevations. Different kinds of rock erode at a variety of rates under particular climatic conditions. As softer rock is worn away, more resistant rock can be exposed to produce landforms. Other landforms develop from volcanic activity or movements along faults during earthquakes. Study of landforms, known as geomorphology, reveals much about the physical and chemical processes acting on Earth’s surface.
Landforms are created by the combined actions of weathering, mass wasting, and erosion. Weathering breaks down bedrock into transportable fragments, mass wasting moves large amounts of material downhill as landslides or rockslides, and erosion transports smaller particles in a number of different ways. Each process can produce characteristic landforms.
The ability of water to move sediment depends on its velocity, which is related to the slope of the bed over which it is flowing. When water is moving rapidly it can transport a great deal, but if water slows it deposits its load. High in the mountains the gradient is steep and erosion dominates. Rivers cut downward and mass wasting adjusts the valley walls to a V shape. These valleys intersect in a branching network as tributaries merge to form a smaller number of larger streams. As the water moves downstream the slope of its bed decreases and eventually a steady state develops where there is little down cutting. Here the slopes continue to retreat until a gentle, rolling topography evolves. Further downstream the slope of the bed is even less, and deposition begins to dominate. The river deposits much of its load during times of flooding, and modifies these deposits the rest of the time. One distinctive land-form resulting from this is the meander. As the river winds back and forth across its floodplain it erodes on the outside of each sinuous curve to form a cut bank, and deposits material on the inside to form a point bar. Eventually the river flows into a standing body of water, either a lake or the sea, and slows down even more, depositing its sediment in deltas. These land-forms emerge if the level of the lake or sea goes down.
A glacier is a flowing mass of ice. It erodes both the sides and bottom of its valley, resulting in distinctive U shaped valleys. The rate at which it erodes is proportional to its depth, because a thicker accumulation of ice bears down harder on the rocks below.
When a glacial tributary joins a larger glacier, the tops of both will usually flow to be at nearly the same elevation, but the bottoms will not be. This difference results in “hanging valleys,” which often display magnificent waterfalls as Yosemite Falls and Bridal Veil Falls in Yosemite Valley, after the ice has melted away. A glacier can transport material at any velocity, but only while it is frozen. Where it melts, its sediments accumulate to form hills called moraines. Sometimes rivers flow beneath the ice, leaving sinuous mounds of sediments called eskers. Other deposits of sediment washed off the top of the glacier form steep sided hills called kames.
Moving air can transport sand and dust, and does not erode solid rock very effectively. Its characteristic landforms, sand dunes, occur in many sizes and shapes. Most have a gentle slope on the windward side, where sand is being eroded away, and a steeper leeward side, where sand is deposited. Movement of sand from one side to the other may result in the migration of the sand dune in the direction the wind is blowing. As with water, transport capability varies with velocity, so sand dunes often develop where the wind slows down. This is the case at Great Sand Dunes National Monument, Colorado, and Death Valley National Park, California, where spectacular, but localized, dune fields occur.
Although erosion and deposition are most easily observed where solid sediment is being moved about, invisible chemical reactions also produce landforms. As water moves through the soil it becomes acidic, in part because of the addition of carbon dioxide produced by the decay of organic matter. This weak acid is able to dissolve some kinds of rock, particularly limestone, giving us spectacular underground caverns, such as Mammoth Cave, Kentucky. In other cases, for example Carlsbad Caverns in New Mexico, naturally occurring sulfuric acid is responsible for limestone dissolution. If caves develop near the surface they often collapse, and a landform called a sinkhole develops above the cave-ins. The distinctive assemblage of landforms associated with the development of caves in soluble rock is known as karst topography.
After passing through limestone, the water often becomes saturated with calcium carbonate. If it comes to the surface in springs, the calcium carbonate may precipitate to build up mounds of travertine, such as those in Saratoga Springs, New York. Similar travertine deposits can develop in arid climates when evaporation brings about the precipitation of calcium carbonate. Sometimes these form a series of little dams holding back pools of water, such as at Mooney Falls in the Grand Canyon, Arizona.
As erosion progresses, by whatever means, some rock units will be more resistant than others. These will be left exposed at higher elevations, and may protect underlying rock. Depending on the orientation and shape of the resistant unit, and the agents of erosion acting on it, various landforms may develop.
Although easily dissolved by groundwater in humid regions, limestone is a resistant rock in arid areas. Its crystalline structure gives it strength, much like solid lava. Where nearly horizontal, both of these rock types are found capping, and protecting, softer rocks beneath them in much of the American southwest. When erosion breaches the resistant unit, it may cut down through the soft rock below very rapidly, leaving isolated islands of resistant cap rock. As this continues, the protection these cap rocks provide preserves tall, nearly vertical landforms called buttes, if they are small, or mesas if they are larger. Monument Valley, near the four corners of Utah, Arizona, New Mexico, and Colorado has dozens of spectacular examples.
If a resistant layer of rock dips at a moderate angle, it will often hold up an asymmetric hill. A small asymmetric hill underlain by layered sedimentary rocks is called a hogback, and a larger one is a cuesta. Rivers, cutting down through such layers, leave a notch as they cut. If a river continues to flow through such a notch it is called a water gap; if not, it is called a wind gap. Delaware Water Gap, between New Jersey and Pennsylvania, is a classic example.
A resistant unit may also be nearly vertical, in many cases because it is formed that way. Molten rock can flow into vertical cracks deep beneath the surface, and then solidify into resistant igneous rock bodies called dikes. When surrounding rocks weather away, the resistant rocks can form vertical walls extending many miles. The Spanish Peaks of south central Colorado have well-known swarms of such dikes dominating the topography.
Vertical walls can form from resistant sedimentary units which have been rotated into a vertical orientation. Seneca Rocks, in West Virginia, is the remnant of a resistant sandstone unit which is now vertical.
If nothing countered weathering and erosion, the continents would be reduced to sea level in a few million years. Tectonic processes, driven by the gradual movements of the tectonic plates comprising Earth’s lithosphere, raise parts of the continents and produce their own landforms.
Volcanism produces a number of landforms. First, of course, are volcanoes themselves. These may be steep sided cinder cones, gently sloping shield volcanoes constructed of lava flows, or a combination of the two, called composite volcanoes, or stratovolcanoes. Cinders and ash fall out of the air and accumulate in steep-sided piles, but these are easily washed away by agents of erosion so they are rarely very large. Solidified lava flows are much more resistant to erosion, but because the lava flows downhill easily before it cools, their slopes are usually very gentle. The composite volcanoes have layers of ash, giving them substantial slopes, protected from erosion by layers of lava. Many of the most famous volcanoes, such as Mt. Fuji, are composite.
Like any fluid, molten lava flows downhill, moving down valleys and off ridges. Frequently it will cool and solidify in the valleys, forming a rock that is very resistant to weathering and erosion. As time passes, the surrounding softer rocks may be eroded away, leaving the lava flows at a higher elevation, protecting the rock beneath them from erosion. In this way so-called inverted topography is developed, where those areas which were lowest become the most elevated.
As tectonic plates move, bending and twisting within them produces fractures called faults. Where these reach the surface they can produce scarps—sharp changes in elevation—if movement on the fault had a vertical component. Scarps can be very small, or the size of mountain ranges. If a typical mountain range is cut by a fault with large vertical movement, many ridges may be beveled off along the same plane, giving rise to what is often called a faceted mountain range. One classic example is the Grand Teton range in Wyoming.
If rocks on either side of a fault move horizontally, such as on the San Andreas fault, deformation of the rock in the vicinity of the fault may make it susceptible to weathering and erosion. This can result in long, linear valleys such as those in much of southern California. These valleys, if filled with water, become sag ponds, such as the San Andreas Lake. Dry valley floors are often among the flattest terrain, making
Deposition —Accumulation of sediments at the end of their transport by erosion.
Erosion —Movement of material caused by the flow of ice, water, or air, and the modification of the surface of Earth (by forming or deepening valleys, for example) produced by such transport.
Fault —A fracture in Earth’s crust accompanied by a displacement of one side relative to the other.
Joint —A fracture in bedrock across which there has not been significant displacement, but that forms as a result of extensional stresses.
Weathering —Biological, chemical, and mechanical attack on rock which breaks it up and alters it at or near the surface of Earth.
them prime locations for building municipal facilities. As the reason for their existence has become understood, however, the wisdom of such construction has been called into question.
Sometimes sets of fractures develop where the surface of Earth is stretched. Such fractures have no displacement parallel to the fracture, as do faults, and are called joints. Weathering, particularly in arid regions, may exploit these joints, leaving a series of vertical slabs of rock. Continued weathering of these slabs can result in the formation of arches, such as those at Arches National Park, in Utah.
In addition to producing its own distinctive land-forms, deformation of Earth’s crust is influential in controlling what landforms result from differential weathering and erosion. During mountain building episodes, large volumes of rock are compressed, folded into complex three-dimensional forms, and sometimes metamorphosed under intense heat and pressure into different kinds of rocks. Later, when these folded layers are exposed to the agents of weathering and erosion, the more resistant units become ridges that outline the folds. Often resistant units offer better protection at the bottom of the fold than they do at the top, resulting in landforms with higher elevations over what were the troughs in the folds, and lower elevations over what were the crests. Much of the valley and ridge areas of Pennsylvania and adjacent states have this kind of landform.
Bloom, A.L. Geomorphology: A Systematic Analysis of Late Cenozoic Landforms. 3rd ed. Long Grove, IL: Waveland Press, 2004.
Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2004.
Otto H. Muller