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. . Plateau. The Four Corners region is characterized by a low, rolling landscape and broad river valleys, with cliffs where a hard-over-soft rock couplet exists. These cliffs retreat down the structural slope with drainages formed on softer rock layers. Elsewhere on the plateau the river system runs through canyon country, a highly dissected terrain with great relief and short, steep tributaries. The Colorado Plateau contains numerous monoclines, laccolithic intrusions and other structural features and the various tributaries and river has carved a number of spectacular canyons through these landscapes, appearing mature in the Four Corners region, but young and actively eroding in the canyon country. The Colorado River system cut through, rather than around, several major clinal features (Fig. 5). The upper Green River carved through alternating soft and more resistant strata in an enigmatic course (U.S. Bureau of Reclamation 1968). The San Juan River channel passed directly through the Hogback monocline in northwestern New Mexico and the Rapley monocline in southeastern Utah. The Colorado River cut through the Kaibab anticline in Grand Canyon. Thus, the river either selected its course before the region became deformed or incised its channel through overlying soft sediments and was thereby restricted to its arduous and complex route through sedimentary terrain. The course of the Colorado River's channel has stimulated more than a century of debate among geologists. Powell (1895) sug~ested that the river's channel was antecedent to regional upllfting and simply cut through a rising Colorado Plateau; however, recent geomorphological research suggests that the eastern and western portions of the drainage became integrated at different times. Others have postulated superposition of the channel by headward erosion, or a combination of these two ideas (Hunt 1956). The eastern Colorado River Basin. Hunt (1956) found evidence of an early Cenozoic, south-directed Colorado River along the western flank of the Rocky Mountains in the eastern Colorado Plateau, which flowed to an unknown destination. This portion of the plateau is dominated by the upper Colorado and Green River drainages which have rather separate and complex histories. The Colorado River. The drainage of the upper Colorado River became integrated in the last 10 m.y. Upstream of Rifle, Colorado, the river and its tributaries have cut down through three datable groups of basalts that are locally interbedded with sediments. These strata reveal that little tectonism occurred from 24 m.y. to 10 m.y. ago (Larsen et al. 1975). The graben basins between the older Laramide uplifts filled with erosional debris and lava flows, producing a landscape with low relief. . . . . . . . . 7 .