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 deformed--if they are sedimentary--and 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 w...
