Hillslopes are a fundamental unit of a landscape, comprising that reach of ground between a drainage divide and a valley floor, and thus much effort has been expended in their study. Early research by Davis (1899) and Penck (1924) was directed toward developing unified theories of slope formation and evolution. Subsequently, emphasis has shifted toward morphometric description of slopes (Strahler, 1956) and study of hillslope processes (Schumm, 1956). Currently, geomorphologists are making impressive advances in understanding hillslope forms and processes (Carson and Kirkby, 1972; Scheidegger, 1970, 1975; Huggett, 1985). While unified theories of hillslope formation and evolution are apparently many years away, the next generation of models can be based on carefully obtained measurements of hillslope processes.
Slope movements of several different types are among the principal processes by which hillslopes evolve. Slope movements are downward and outward movements of slope-forming materials composed of natural rock, soils, artificial fills, or combinations of these materials. This definition is identical to the definition of landslide used by Eckel (1958a). The terms landslide and mass wasting are sometimes used synonymously for slope movement. Slope movements, however, include some processes that involve little or no true sliding, such as falls and flows, and do not include some processes contained in mass wasting such as subsidence (Sharpe, 1938).
Progress in understanding and control of slope movements has been the result of a truly interdisciplinary effort involving geological scientists, engineers, physicists, and hydrologists. Most of the major practitioners in applied geology in the 19th and 20th centuries have contributed significantly to our understanding of slope movement types and processes.