The purpose of this report is to describe the lower Upper Cretaceous strata in the Bighorn Basin, emphasizing evidence of major unconformities and the temporal and spatial distribution of those unconformitied. Data used in this investigation were obtained from 13 measured sections, about 50 fossil localities, and the electric logs from about 210 wells. Of the well logs examined, 20 are represented on the illustrations in this report. The locations of the wells and measured sections are shown on Figure 1 and described in Table 2.