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    <dct:description>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. This set of slides includes examples of wave erosion, wind and water erosion, valley shapes, and glacial erosion. The views are often dramatic. Many were taken at U.S. National Parks and Monuments. 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 weatheringand 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. 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, creating 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 insculpting 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 SystemsA 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 hillsand 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. GlaciationA prerequisite for glacier formation is that more snow accumulates than melts in the period of a year. Ice sheets have covered major portions ofthe continents, and valley glaciers have formed and melted in many of the mountainous regions of the globe. When these glaciers melted, they haveleft 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.</dct:description>
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        <dcat:keyword>weathering</dcat:keyword>
        <dcat:keyword>earth science</dcat:keyword>
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        <dcat:keyword>seismic activity</dcat:keyword>
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        <dcat:keyword>wind erosion</dcat:keyword>
        <dcat:keyword>wind erosion</dcat:keyword>
        <dcat:keyword>marine environments</dcat:keyword>
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        <dcat:keyword>coastal erosion</dcat:keyword>
        <dcat:keyword>terrestrial ecosystems</dcat:keyword>
        <dcat:keyword>soils</dcat:keyword>
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        <dcat:keyword>u.s. department of commerce</dcat:keyword>
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