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4 delivered by the Little Snake River entering the Yampa just four miles east of the Monument Boundary. This sediment load is transported through the cobble substrate reaches in the lower half of the canyon. Dominating the landscape of Dinosaur National Monument are the Green and Yampa River canyons. Both rivers have been entrenching into bedrock since late Cretaceous time, about 70 million years (m.y.) ago. Prior to that time they were meandering rivers flowing on a mature plateau. The rivers still meander, but have incised within steep-walled canyons. The mechanics of meander entrenchment are not clearly understood, but W. R. Hansen has a good theory for incasement of the Green River (1969, and personal communication, 10/7/83). He postulates that the Lower Green originally flowed from Browns Park into the North Platte River on the then-developing Browns Park Formation. Sediments and volcanic ash were accumulating to a thickness of over 7,000 feet. Between the Uinta and Yampa Faults the eastern part of the Uinta Mountains was collapsing. The Browns Park Formation overtopped the valley rim, and the Green River breached the Uinta crest at the present site of Lodore Canyon. The river cut quickly through the soft Browns Park formation to harder rocks below. The main Green was still flowing eastward across the rising Continental Divide. The Lower Green incised itself to its new base level, and eventually captured the main Green and turned it south. The Yampa River appears to have a similar history of incision into the Browns Park formation. The Yampa canyon profile varies distinctly between reaches with different rock lithologies. Two formations are seen most often in the canyon, the Morgan and the Weber. The Morgan consists mainly of limestone beds and underlies the Weber. The Weber is a relatively soft sandstone. Where the Yampa flows through the Morgan, the valley profile tends to be asymmetrical. On the south side steep walls are found, while the north side slopes gently and is covered with talus. The asymmetry is caused by lithology and by the dip of the beds, which is 7 to 10 degrees to the southwest. The lower member of the Morgan is an incompetent shale, which rests on the Round Valley Formation, a limestone. Where exposed by the river the shale slides on the Round Valley downslope, causing overlying rocks to collapse. Most landslides therefore occur on north slopes, in the Morgan. The river channel is confined between the steep talus slopes and has no floodplain. Channel location in the valley has been dictated in some reaches by the ancient landslides. In the Weber formation the canyon's profile is symmetrical. The Weber is a soft sandstone which is easily eroded by the river. Smooth, curving walls are often vertical or past vertical depending on the length of time they are subjected to the erosive forces of the river. Channel bed slope varies with rock type encountered. In the upper reach of the canyon, the slope is steep where the river flows through the Morgan. Boulders and talus from slides armor the river bed slowing its rate of downcutting through the hard limestone formation. The slope