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The Hydrologic System
The hydrologic system, which includes all possible paths of motion of Earth's near-surface fluids including air and water, is largely responsible for the variety of landforms found on the continents. Heat from the sun evaporates water from oceans, lakes, and streams. Although most of the water returns directly as precipitation to the oceans, some of the water is precipitated over land as rain or snow. If it is precipitated over land, it then begins its journey back to the sea as "runoff." The relentless action of surface runoff, streams and rivers, glaciers, and waves sculpts the rock into intriguing and bizarre shapes.

Water Erosion of Horizontal Strata in Semiarid Lands After the horizontal strata in today's semiarid landscapes were deposited, they were uplifted, twisted and cracked, forming joints-parallel fractures in the brittle rock. These joint systems are made vulnerable to weathering and frost wedging by the erosion of the overlying resistant layers. As the joints and fractures widen, rock fins are produced. In addition to fins, large flat areas called plateaus may be eroded along joints into smaller flat topped mesas and still smaller buttes. Buttes are further eroded into pillars and pinnacles.

Slabs of rock may break away between two joints in a fin so that an alcove (a recess) forms. As the alcove enlarges, a small window may be produced in the cliff face. Weathering then proceeds inward from all surfaces, and as weathering removes the rock surface, pressures locked within the formation itself are released, breaking off more rock flakes. Rock falls from the ceiling of the opening, and the span thins and elongates. These erosive forces-dissolution, frost action, and release of compression-eventually enlarge the window in the fin, and creates an arch. Variability in the cementing materials and the rock structure in the arch floor, buttresses or ceiling determines the size, shape, and age of the arch. The shape and size of the arches changes over time, and the forces that created an arch finally destroy it leaving goblin-like columns.

Wave Erosion
Coastal areas are bombarded by water in constant motion. Although wind and/or storm-generated waves, tides, and tsunamis all play a role in sculpting the shoreline, the relentless motion of waves is perhaps the most important of these factors. These waves are generated by the wind at sea. As the wave approaches the shore, it breaks, and the surf surges on shore causing erosion, transportation, and deposition in beach areas. The breaking waves transporting sand and gravel encounter the headlands and powerfully abrade them horizontally forming platforms, cliffs, alcoves, and caves in the rock.

River Systems
A river system functions as a unified whole, adjusting its profile to establish equilibrium among the factors that influence flow. These factors include discharge, velocity, topographic gradient, base level and load. Although downcutting by the stream is slow on resistant rock units, it occurs more rapidly than erosion and mass movements of the slopes. As a result, vertical-walled canyons develop. But if slope processes keep pace with downcutting, the landscape is characterized by smooth rolling hills and valleys. Initial dissection and slope retreat occur as a result of uplift. Slope retreat causes non-resistant rocks to recede from the river so that terraces are left on resistant rock layers.

Glaciation
A prerequisite for glacier formation is that more snow accumulates than melts in the period of a year. Ice sheets have covered major portions of the continents, and valley glaciers have formed and melted in many of the mountainous regions of the globe. When these glaciers melted, they have left behind an altered landscape. The glaciers alter pre- existing river patterns. They scour mountain tops, valleys, and continental surfaces, transport the eroded particles, and finally leave behind the removed material as glacial deposits.

Whether it is churning in the waves of the sea, falling as rain and flowing over arid lands, flowing in river systems, or frozen in glacial systems, water in its various forms sculpts the land into fascinating shapes.

	Headlands and arches on Oregon coast WAVE EROSION  Headlands, Arches, Sea Caves, and Stacks Oregon Coast Wave refraction concentrates energy on headlands where zones of weakness, such as joints, faults, and non-resistant beds, erode more quickly, producing sea caves in those areas. The sea caves enlarge to form sea arches. Both sea caves and arches of various sizes can be seen in this view of the Oregon coast. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-01
	Headlands and sea stacks on Oregon coast WAVE EROSION  Headlands, Arches, Sea Caves, and Stacks Oregon Coast Eventually an arch collapses, leaving a sea stack on the remaining headland. Sea stacks are visible at the ends of promontories on this stretch of the Oregon coast. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-02
	Alcove in canyon wall, Lake Powel, Arizona WIND AND WATER EROSION  Arches and Bridges Alcove at Lake Powell, Paige, Arizona Here an alcove has formed in the wall of the canyon containing Lake Powell. This erosion feature was in place prior to the construction of the Glen Canyon Dam which produced Lake Powell, and improved access to this feature. Note the scars left when rounded slabs of rock were spalled from the wall leaving overhangs in a cliff face. One might ask whether wind has played a major role in sculpting such features. While the frequent winds lift tons of sand, most of the grains are lifted only a few feet above the ground. It is water, even in these semiarid climates, which plays the major role in the sculpting of these massive erosion features. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-03
	WIND AND WATER EROSION Arches and Bridges Delicate Arch, Arches National Park, Moab, Utah The rock in this arch was once a part of an ancient fin. An alcove formed in the fin (as in slide #3). Erosional forces continued until this free-standing arch perched at the edge of a slick-rock bowl is all that remains of the fin. Differences in the rock's resistance to erosion make the arch appear to resemble a dragon sniffing a flower. Note the resistant cap rock that is still in place. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-04
	Balancing Rock, Moab, Utah WIND AND WATER EROSION Arches and Bridges Balancing Rock, Arches National Park, Moab, Utah Over time the shape and size of the arches change, and the forces that created an arch finally destroy it, leaving goblin-like columns. In this case a layer of more resistant rock caps the formation. Earthquakes can cause balanced rocks to topple, so the existence of balanced rocks suggest tectonic stability. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-05
	Rainbow Bridge, Arizona WIND AND WATER EROSION Arches and Bridges Rainbow Bridge, Paige, Arizona A natural bridge is begun when a river's meandering course creates a bend that nearly doubles back on itself. Water washes into and scours the inside wall of the canyon. After the river or stream cuts an opening through the narrow wall, the stream may change course taking a short cut under the bridge and eliminating the meander. The same erosional forces that enlarge arches also work to expand the opening under the new natural bridge. In this view a small canyon has been cut by the stream below the pillars of Rainbow Bridge. [Photo credit:John Lockridge, plockridge@mho.net.] File:Erosion-06
	Pothole, Paige, Arizona WIND AND WATER EROSION  Arches and Bridges Pothole, Glen Canyon, Paige, Arizona Potholes result when pebbles borne by streams (in this case an intermittent stream) are trapped in a depression and swirled around by currents. The rotational movement of the sand, gravel, and boulders acts like a drill cutting deeply into solid rock. As the pebbles and cobbles are worn away, new ones are carved from the hole and continue the drilling into the floor of the stream channel. The pothole shown is about two and a half feet deep. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-07
	Cross-bedding, Zion National Park, Utah WIND AND WATER EROSION  Cross-bedding Checkerboard Mesa, Zion National Park, Utah This formation is composed of ancient sand dunes, produced when winds, blowing across an ancient desert, piled up grains of sand. Over time, the dunes shifted and were reworked. Eventually calcium-bearing solutions cemented them in place. The resulting formation seen in this Navajo sandstone is called "cross-bedding." The frozen dunes reveal that the wind direction was from north to south (from left to right, in the photograph). The "checkers" that groove the surface of this formation are formed by the weathering of horizontal bedding planes and vertical cracks. Vertical cracks and joints in the sandstone were created in the stress of uplift. As water is channeled into the crevice, it dissolves the cement holding the rock particles. The cement may be redeposited in another location hardening the rock in that area. Other physical cracking results from the freezing and expansion of the water. [Photo credit: John Lockridge, plockridge@mho.net.] File:Erosion-08
	Cliff erosion, Bryce Canyon, Utah WIND AND WATER EROSION  Cliff Erosion Bryce Canyon National Park, Utah Eroded columns, pinnacles and formations in Bryce Canyon National Park, Utah. The delicate coloring and formations are produced by chemical weathering. This form of weathering occurs wherever seeping ground water circulating through the sandstone dissolves the calcium-carbonate cement between the individual grains, causing the rock to crumble to sand. Cavities work their way back into the walls of the escarpment. Over time, the cavities deepen and roofs collapse. Columns are formed which, in turn, finally topple. Pink, white, yellow, and red formations are produced by oxidation (chemical weathering) of the iron-bearing minerals found in the rock. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-09
	Canyonlands, Moab, Utah WIND AND WATER EROSION  Cliff Erosion Canyonlands, Moab, Utah Erosion shapes the sandstone into plateaus, mesas, buttes, pinnacles, and gullies-all are visible in this image. Where erosion is rapid, little soil is left and plants cannot grow. This sort of landscape has a smooth appearance, resulting in the name "slickrock desert." Note the white deposit known as desert pavement. Differential erosion has produced alternating cliffs and slopes. Cliffs form on resistant sandstone and limestone formations; slopes develop in nonresistant shale. Talus slopes abound where weathering of the fins and columns has proceeded more rapidly than removal of the eroded material. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-10
	Whittier Trail, Canyonlands, Utah WIND AND WATER EROSION Cliff Erosion Whittier Trail, Canyonlands, Utah The narrow jeep road traverses the canyon wall and the slope debris (called talus). In areas where downcutting outstrips the removal of the products of weathering, vertical-walled canyons develop. Rock fragments produced by frost action and weathering fall from the rock face, then tumble or creep down the slope were they accumulate as slope debris. Continued weathering transforms the talus into smaller and smaller particles. Gravity and/or running water move the particles down the slope until they are fed into a stream and carried away. [Photo credit: John Lockridge, plockridge@mho.net.] File:Erosion-11
	San Andreas Fault, California WIND AND WATER EROSION Cliff Erosion San Andreas Fault This photo shows the San Andreas fault at a road cut on California Highway 14. About two thirds of the way from the bottom to the top is a line. Below the line is the road cut showing non-eroded, convoluted layers. Above the line, the convolutions have been exposed in relief by weathering. Faulting along the San Andreas is visible in the lower left of the photo. [Photo credit: Clifford E. Harwood, Encino, California.] File:Erosion-12
	Devils Tower, Wyoming WIND AND WATER EROSION Cliff Erosion Devils Tower Wyoming The exposed rocks at Devils Tower National Monument are classified into two general types: igneous and sedimentary. The tower itself is composed of igneous rock formed by cooling and crystallization of once-molten materials. The rocks exposed around the tower are sedimentary-layers of shale, sandstone, silt stone, mud stone, gypsum, and limestone. They were formed by the consolidation of fragmented materials derived from other rocks or the accumulation of chemical precipitates that were deposited on the floors or near the shores of ancient seas. Devils Tower owes its impressive height to the differing rates of erosion of these two rock types-the soft sedimentary rocks have eroded more readily than the hard igneous rock. One of the most striking features of the tower is its polygonal columns, formed as the igneous mass slowly cooled and crystallized. Most of the columns are 5-sided, but some are 4-sided or 6-sided. Numerous cross-fractures in the upper part of the tower divide the columns into many small, irregularly-shaped blocks. [Photo credit: Michelle Flores, National Geophysical Data Center.]  File:Erosion-13
	V-shaped valley, Zion National Park, Utah VALLEY SHAPES V-Shaped Valley The Narrows, Zion National Park Trapped between the nearly vertical walls of the gorge, the Virgin River flows through the Narrows that it has, over time, gouged from rock. Flash floods hasten the widening and deepening of the canyon. The cement that holds together the rounded grains of quartz in these rocks is readily dissolved by rain that wets the walls and by water that seeps through the rock, loosening tiny particles and permitting them to fall or be swept from the cliff face by showers and wind. Sandstone is reduced to sand, and then is easily removed by streams. [Photo credit: John Lockridge, plockridge@mho.net.] File:Erosion-14
	Mature Grand Valley, Colorado VALLEY SHAPES Mature Valley Grand Valley (Fruita, Colorado) from Colorado National Monument This view looks north from Colorado National Monument across the Grand Valley of the Colorado River, to the Roan Cliffs, and the Book Cliffs. The main view is a mature valley cut by the Colorado River. Terrace levels indicate stages at which the river stabilized for a time and widened its flood plain before beginning another cycle of downcutting and erosion. Landslides form in the talus below freshly scarred cliffs, as erosion of soft shale layers in and below undermines the overlying sandstone layers. Weathering and slope processes continuously widen the valley. Where it is not protected by the sandstone caprock, the shale erodes into humpbacked badlands visible in the distance. The tributary valley in the foreground has retained its steep canyon-like walls, indicating that downcutting is still occurring faster than slope processes that widen and soften the canyon features. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-15
	U-shaped valley, Alberta, Canada VALLEY SHAPES U-Shaped Valleys (Glaciation) Ice Fields Parkway, Alberta, Canada After the glacier occupying a valley melts, the valley displays a steep U-shaped cross section. The walls of the valley are plucked clean but moraine material is left along the sides of the valley (lateral moraine), and in the region of the farthest reach of the glacier (terminal moraine). In this view talus can be seen on the slopes, and mounds (tree- covered moraines) are visible. Stream meanders are visible on the valley floor, which has been cut nearly level by the action of the glacier. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-16
	U-shaped valley, Rocky Mountain National Park, Colorado VALLEY SHAPES  U-Shaped Valleys (Glaciation) Lake Mills, Rocky Mountain National Park, Colorado This valley is one that was greatly modified by glaciers during the last ice age. A valley glacier commonly fills more than half of the valley length, and as it moves, it modifies the former V-shaped stream valley into a broad U- shaped or trough-like form. The head of the valley is sculptured into a large amphitheater called a cirque (visible in the middle background). Where several cirques approach a summit from different directions, a sharp, pyramid-shaped peak called a horn is formed (middle background). The projecting ridges and divides between glacial valleys are subjected to rigorous ice wedging, abrasion, and mass movement. A knife-edged ridge (arete) is the result of glaciers coming together from opposite directions (from left to middle of photo) Moraine material composed of rock fragments is created from glacial erosion. A truncated spur is visible on the right side of the photo just above the tree-covered ridge. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-17
	Glacial erosion, Grand Tetons, Wyoming GLACIAL EROSION Grand Tetons, Grand Teton National Park, Wyoming In fairly recent geologic times, slippage began to occur along a fault in the rocks. A large block of earth, 15 miles from east to west and 40 miles from north to south began to move upward. The block rose most rapidly on the east side and so was tilted to the west. As the Teton Range was being uplifted, water and frost wedging began to remove the topmost layers and expose ancient gneisses and schists. V- shaped valleys were formed. Then a change in the climate brought the onset of the Ice Ages. Glaciers sculpted the peaks and carved U-shaped valleys, leaving the mountains in their present magnificent form. A glaciated valley is visible on the left in this view. It still has a glacial remnant in it. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-18
	Hanging Valley, Yosemite National Park, California GLACIAL EROSION Yosemite National Park Upper Yosemite Falls drops from its "hanging" valley into the Yosemite chasm. These hanging valleys and waterfalls (among the grandest collection of waterfalls in all the world) were left after glacial ice scoured out the main valley to a depth of 2,000 or more feet. When the glacial ice in the main valley melted, the tributary valleys were left "hanging" high above the main valley floor. The lower slopes have been scoured smooth by the glacier while the upper slopes above the ice line remain rugged (upper left). [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-19
	Glacier in Ice Fields Parkway, Canada GLACIAL EROSION Ice Fields Parkway, Alberta, Canada Glacial crevasses (deep cracks in the ice) open from the downslope movement of the glacier and generally form perpendicular to the direction of glacial movement. As glaciers move, they quarry and pluck away at the lead surface on valley floors and along the sides. They also erode headward into mountain sides, carving steep but rounded valley headwalls called cirques (middle background). These cirques form as glacial ice flows down hill, and a crevasse forms between the glacier and the rock headwall. In summer, melt water flows between the glacier and the headwall then refreezes, rejoining the glacier to the headwall. As the glacier moves away from the headwall, it pulls away pieces of the rock headwall with it, enlarging the cirque. [Photo credit: John Lockridge, plockridge@mho.net.]  File:Erosion-20