During Late Pennsylvanian time a major landmass, the Ancestral Front Range, extended north and south through central Colorado. Terrigenous sediments in the adjacent basins were, for the most part, derived from this highland. These sedimentary rocks are predominantly redbeds which have been interpreted to be of primary detrital origin, and the conclusion has been drawn that the red pigment (hematite) originated in lateritic soils which covered the highland. Accordingly, because modern laterites are formed only in areas of tropical climates, it has been concluded by various authors that the climate in the area of the Ancestral Front Range during Pennsylvanian time was warm and humid.

The close association of redbeds with evaporites in the Eagle basin on the west side of the Front Range suggests an arid climate. Hence, two conflicting lines of evidence exist concerning the Pennsylvanian climate. This study was undertaken to determine what information concerning this problem might be obtained from a study of the clay mineralogy of the rocks which were derived from the Ancestral Front Range. Inasmuch as most of the clay-size material in the rocks probably was derived from the soils of the source area, and because climate has a dominant effect on the types of clay minerals formed in soils, it seems likely that the nature of the clays would offer important clues regarding the source-area climate.

Clay-mineral analyses were made of samples from the Molas regolith, the Belden Formation, and the Minturn Formation on the west flank of the Ancestral Front Range; and from the pre-Fountain regolith, the Glen Eyrie Formation, and the Fountain Formation on the east flank of the Ancestral Front Range. Two major types of clay-mineral assemblages occur in these rocks. The Molas regolith, and the Belden and Glen Eyrie Formations contain illite, montmorillonite, mixed-layer illite-montmorillonite, and significant amounts of detrital koalinite. On the basis of this assemblage it is concluded that the climate in which these clay minerals formed was temperate with moderate rainfall. The pre-Fountain regolith (east flank of the Ancestral Front Range), and the Minturn and Fountain Formations, co tain illite, mixed-layer illite-montmorillonite, chlorite, mixed-layer chlorite-vermiculite, and subordinate amounts of authigenic kaolinite. On the basis of this clay-mineral assemblage it is concluded that the climate in which these clay minerals formed was semiarid to arid.