The Green and Yampa River Canyon country within the boundaries of Dinosaur National Monument represents the deeply dissected eastern end of an east-west anticlinal fold, the Uinta Mountains, and its accompanying smaller flank folds. Oldest formation exposed in this area is the Uinta Mountain group (Algonkian) and the youngest, Browns Park formation (Pliocene). Along the crest of the main arch, in the deeper canyons, and at some places along the upthrown sides of faults are exposed massive dark red blocky-weathering quartzitic sandstone, shale, and conglomerate beds of the Uinta Mountain group, and the shale and quartzitic sandstone beds of the Lodore formation (Cambrian). In much of the area, Mississippian, Pennsylvanian, and Permian rocks form long dip-slopes toward the outh.

Typical of Uinta Mountain structure and representative of the smaller folds that flank the main arch of the Uinta Mountains is the Split Mountain-Yampa Plateau area. Formations along the crest of this anticlinorium have low attitudes in contrast to the sharply flexed, steeply dipping, younger flanking formations. These strata with their well developed flat-iron and race-track patterns of exposure exhibit a striking, colorful geological cross section ranging in age from Lower Pennsylvanian (Morgan) to Upper Cretaceous (Mancos). Lower Pennsylvanian and Lower Mississippian are exposed in the deeper part of Split Mountain Canyon.

High-angle normal faults occur in the Monument area. The Split Mountain-Yampa Plateau section is traversed by the Yampa fault which seems to die out in the western part of the fold. Split Mountain Canyon has been carved by the Green River along the western end of this zone.

Stream piracy and deep entrenching of the meandering stream courses during Pleistocene have produced a spectacular scenic area.