STRUCTURAL LANDFORM

any topographic feature formed by the differential wearing away of rocks and the deposition of the resulting debris under the influence of exogenetic geomorphic forces. Such forces operate at the interface of the planetary atmosphere, lithosphere, cryosphere, and hydrosphere. The processes generating these forces are the major agents of erosion, transport, and deposition of debris. They include fluvial, eolian, glacial, groundwater, and coastal-marine processes, as well as those associated with mass movement. Structural landforms result from forces generated by such processes interacting with the resistances imposed by rocks and sediments. For change to occur, the forces must exceed the thresholds of resistance imposed by the earth materials on which they act. The landform itself, however, may alter the forces by developing specific shapes. Sand dunes, beaches, river valleys, and glacial drumlins are all examples of landforms that modify the forces imposed upon them. Such self-regulation of landform development is a quality of landscapes that achieve equilibrium. Although structure and lithology establish the resistance factors for structural landforms, climate defines the nature of the exogenetic geomorphological processes. In cold regions ice-related processes dominate in the development of landscapes, while in warm-wet regions fluvial processes exert primary control. Thus, a climatically controlled style of landscape development is imposed on the structurally defined surface. Moreover, process and structure interact through geologic time on an evolving landscape. As pointed out by the eminent William Morris Davis, landscape is a function of the trilogy of structure, process, and time. Victor R. Baker Additional reading Landforms resulting from erosion and deposition are analyzed in Richard J. Chorley, Stanley A. Schumm, and David E. Sugden, Geomorphology (1984); and Dale F. Ritter, Process Geomorphology, 2nd ed. (1986). Victor R. Baker