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Four-level Braun-Blanquet vegetation type hierarchies of an arctic (a section of the Arctic Coastal Plain at Atkasook, Alaska) and an alpine (Indian Peaks area, Colorado Southern Rocky Mountains) area were compared through their vascular plant taxa composition. Vegetation types at all hierarchy levels were more numerous, less well defined, and less clearly distributed along the controlling environmental gradients in the Arctic than in the alpine. This greater environmental and vegetational heterogeneity and relatively poor definition of vegetation types are probably controlled by the surface disturbances of the thaw-lake cycle, rivers, cryoturbation, and wind, which cause significant horizontal movement of materials...
Since the beginning of the Cenozoic period several hundreds of metres of the sedimentary cover have been removed from the Colorado Plateau. Palaeoclimatic considerations show that the Colorado Plateau has been dominated by dry climates throughout the Cenozoic with the possible exception of the early Palaeocene. Today in the still prevailing arid climate, which strongly accentuates differences in rock resistance, the relief shows a structurally controlled cuesta scarp topography in the slightly deformed strata of alternating resistance. In examining whether the denudational efficiency of scarp retreat was sufficient to account for the wide erosional gaps in the sedimentary cover, rates of scarp retreat were determined...