The Colorado Piedmont, adjacent to the Front Ranges, contains broad interstream surfaces which stand above the modern valley bottoms, but in striking contrast to the steep mountain front and the aligned hogbacks of the foothills. The interstream flats occur both in abutting and detached relationship with the mountain front. Often a multiple arrangement is displayed, and several steplike levels are preserved. Merging of adjacent levels is common, particularly upstream. Remnants of higher surfaces, as well as ridges of resistant rock, project above the surfaces. On many, old stream channels converge toward a major canyon in the mountain front.
The piedmont surfaces are fanlike terraces floored with coarse fanglomerate deposits. The alluvium overlies bedrock erosion surfaces which truncate the tilted strata of the foothills belt, often extending onto the rocks of the mountain mass. Ordinarily the erosion surfaces disregard variations in rock type. The chief irregularities in the bedrock surfaces are broad, shallow depressions and small, channel-like incisions.
Wide, thinly alluviated valleys with relatively flat bedrock floors occur below the piedmont interstream surfaces. The width of these valleys correlates with the resistance of the rock types traversed. Some of the valleys originate in the mountain mass and extend through the piedmont, whereas others, tributary to the mountain-born drainage lines, begin in the piedmont. Most are dry throughout the year, except during occasional summer storms. Valley segments in the mountain area are deep and narrow, except locally where small, thinly alluviated, flat-floored basins are present (Tator, 1949, p. 1774–17831.) The piedmont valleys contain flights of rock-floored terraces which form steps below the interstream surface levels.
The origin of the piedmont levels is a major key to the diastrophic history of the Rocky Mountain front region. This present work includes a brief summation of concepts of origin of similar land forms in other areas, detailed descriptions of piedmont interstream surfaces in a selected portion of the piedmont, a brief analysis of the processes of valley widening by sidewall weathering and rock planation by vertical corrasion now active in the modern valleys (Tator, 1949), an adaption of these processes to development of terrace and piedmont interstream surfaces in the region, an interpretation of the physiographic history of the area studied, and tentative establishment of middle or late middle Pleistocene and Recent as the intervals during which development of the various surfaces took place.