CONTINENTAL LANDFORM

any conspicuous topographic feature on the largest land areas of the Earth. Familiar examples are mountains (including volcanic cones), plateaus, and valleys. (The term landform also can be applied to related features that occur on the floor of the Earth's ocean basins, as, for example, seamounts, mid-oceanic ridges, and submarine canyons.) Such structures are rendered unique by the tectonic mechanisms that generate them and by the climatically controlled denudational systems that modify them through time. The resulting topographic features tend to reflect both the tectonic and the denudational processes involved. The most dramatic expression of tectonism is mountainous topography, which is either generated along continental margins by collisions between the slablike plates that make up the Earth's lithosphere or formed somewhat farther inland by rifting and faulting. Far more subtle tectonic expressions are manifested by the vast continental regions of limited relief and elevation affected by gentle uplift, subsidence, tilting, and warping. The denudational processes act upon the tectonic stage set and are able to modify its features in a degree that reflects which forces are dominant through time. Volcanism as a syn-tectonic phenomenon may modify any landscape by fissure-erupted flood basalts capable of creating regional lava plateaus or by vent eruptions that yield individual volcanoes. The denudational processes, which involve rock weathering and both erosion and deposition of rock debris, are governed in character by climate, whose variations of heat and moisture create vegetated, desert, or glacial expressions. Most regions have been exposed to repeated changes in climate rather than to a single enduring condition. Climates can change very slowly through continental drift and much more rapidly through variations in such factors as solar radiation. In most instances, a combination of the foregoing factors is responsible for any given landscape. In a few cases, tectonism, some special combination of denudational effects, or volcanism may control the entire landform suite. Where tectonism exists in the form of orogenic uplift, the high-elevation topography depends on the nature of denudation. In humid or glacial environments whose geomorphic agencies can exploit lithologic variations, the rocks are etched into mountainous relief like that of the Alps or the southern Andes. In arid orogenic settings, the effects of aggradation and planation often result in alluviated intermontane basins that merge with high plateaus interrupted or bordered by mountains such as the central Andes or those of Tibet and Colorado in the western United States. In continental regions where mountainous uplifts are lacking, denudational processes operate on rocks that are only slightly deformedif they are sedimentaryand only moderately elevated. This produces broad basins, ramps, swells, and plains. These are most thoroughly dissected in rain-and-river environments (sometimes attaining local mountainous relief on uplifts). Elsewhere, they may be broadly alluviated and pedimented where mainly arid, or widely scoured and aggraded where glacial. Minor denudational landforms are superimposed on the major features already noted. Where aridity has dominated, they include pediments, pans, dune complexes, dry washes, alluvial veneers, bajadas, and fans. Ridge-ravine topography and integrated drainage networks with associated thick soils occur where humid conditions have prevailed. Combinations of these features are widespread wherever arid and humid conditions have alternated, and either category may merge laterally with the complex suite of erosional and depositional landforms generated by continental glaciers at higher latitudes. This article reviews the significant theories of landform genesis developed during roughly the past two centuries. Additional reading The seminal works of William Morris Davis, especially his 1899 essay The Geographical Cycle and his 1905 essay The Geographical Cycle in an Arid Climate, both reprinted in Geographical Essays (1909, reprinted 1954), paved the way for Walther Penck, Morphological Analysis of Land Forms: A Contribution to Physical Geology (1953, reprinted 1972; originally published in German, 1924); and for Lester C. King, Canons of Landscape Evolution, Bulletin of the Geological Society of America, 64:721752 (1953). Davis' thinking continued to dominate geomorphic theory in such textbooks as A.K. Lobeck, Geomorphology: An Introduction to the Study of Landscapes (1939); and William D. Thornbury, Principles of Geomorphology, 2nd ed. (1968). The history of geomorphic theory up to the time of Davis is told in Richard J. Chorley, Antony J. Dunn, and Robert P. Beckinsale, The History of the Study of Landforms, 2 vol. (196473). Cuchlaine A.M. King (ed.), Landforms and Geomorphology: Concepts and History (1976), contains a historical collection of key papers in the development of geomorphic ideas from 1802 to 1972. Introductory essays on the many aspects of the field may be found in Alistair Pitty (ed.), Geomorphology: Themes and Trends (1985).The first comprehensive attempt to synthesize geomorphic thinking on a systems level that incorporates modern tectonic, climatic, and process aspects is H.F. Garner, The Origin of Landscapes: A Synthesis of Geomorphology (1974). An analysis of these and other geomorphic theories to date appeared in William N. Melhorn and Ronald C. Flemal (eds.), Theories of Landform Development (1975, reissued 1980). More restricted aspects of geomorphology are dealt with in Luna B. Leopold, M. Gordon Wolman, and John P. Miller, Fluvial Processes in Geomorphology (1964); G.H. Dury (ed.), Essays in Geomorphology (1966); J. Tricart and A. Cailleux, Introduction to Climatic Geomorphology (1972; originally published in French, 1965); Robert V. Ruhe, Geomorphology: Geomorphic Processes and Surficial Geology (1975); C.R. Twidale, Analysis of Landforms (1975, reprinted 1976); R.J. Rice, Fundamentals of Geomorphology (1977); Dale F. Ritter, Process Geomorphology, 2nd ed. (1986); and Rita Gardner and Helen Scoging (eds.), Mega-Geomorphology (1983). Hessle Filmore Garner