Red-banded early Cenozoic formations in the Rocky Mountain region are principally fluviatile, piedmont and valley-flat deposits which accumulated in a warm, humid lowland environment that prevailed in the tectonic basins.

Differential thermal analyses of the clay complex of twenty-two red and drab mudstone and sandstone layers reveal that illite is the predominant clay mineral group in most of the tested specimens; kaolinite is the principal clay mineral group in four. A small amount of montmorillonite is commonly present.

Chemical analyses of red and drab layers show a higher Fe₂O₃ content in the red beds. Presumably the red pigment is hematite. Inasmuch as brown hydrated ferric oxide is not known to dehydrate to hematite under conditions prevailing at the earth's surface, the hematite in the red layers is interpreted as primary, derived either from older red formations exposed at the basins' margins or from red soil developed on the surrounding uplands.

A review of the lithologic characteristics and stratigraphic relationships of the variegated formations, together with the paleobotanical evidence of a warm, humid early Cenozoic climate, suggests that red upland soil was transported into the basins. Hence, it is postulated that in deposits which accumulated in more open country of a savanna environment the primary red color was preserved, while in sediments accumulating in heavily forested areas drab colors resulted from reduction of the hematite by incorporated plant debris. As the aggrading streams meandered across the basin floors the sites and environments of deposition constantly shifted, thus producing the alternation of lenticular red and drab strata.