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Landslides are abrupt, short-lived geomorphic events that constitute the rapid-motion end of the mass-movement spectrum. The term “landslide” is a misnomer because the process includes gravity displacement of earth material by falling, sliding, and flowing, and most experts also include subaqueous movements. Landslides are also landforms that are nearly ubiquitous in sloping terrain. All that is needed for their formation is a triggering mechanism, such as excessive precipitation, earthquake, or man, to upset natural stability and induce stresses that exceed the shear resistance of the substrate. A new classification is introduced that recognizes the importance various materials play in the production of different landslide types, whether in bedrock, regolith, or sediments. Landslide names also reflect the type of movement and rate, moisture content, shape of the failed surface, and particle size.

There are fewer publications on landslides than on other topics within catastrophic geology, such as earthquakes, volcanoes, and floods. However, the literature is rapidly expanding because of reports required under the southern California grading ordinances and the environmental impact statements mandated in legislation such as the National Environmental Policy Act of 1969. The new awareness and perception are well timed, because landslide damages have been constantly increasing and amount to hundreds of millions of dollars annually in the United States. The escalating costs are the result of increased construction in highways, reservoirs, other service utilities, and urbanization. Double jeopardy occurs in many urban areas where man is forced out of flat areas because of flood hazards and legal prohibitions onto sloping ground that may be landslide prone.

Planning and management strategies for land use in unstable areas are becoming more sophisticated. The Los Angeles grading ordinances of 1952 and 1963 and the San Francisco region five-year investigation by the U.S. Geological Survey are serving as worldwide models. Prevention and

control of landslides require cooperation of engineering geologists, hydrologists, geomorphologists, social planners, and the general public. Recognition of a potential problem is paramount in avoiding disaster. My study of sensitive sediments in the Cowanesque Valley, Pennsylvania, is cited as an example of such preparedness as practiced by the Army Corps of Engineers.

A goal of this chapter is to present an overview and synthesis of the art of understanding landslides. Some problems remain unresolved, such as the processes involved in the movement of rock avalanches. However, rapid advances are being made as new frontiers of space and oceans are explored, yielding new insights into landslide mechanics.

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