Douglass Creek basin lies west of the Continental Divide in the northern part of the Rocky Mountain physiographic province. Numerous minor environmental differences exist between the Douglass Creek area and the Pipestone Springs and Canyon Ferry areas east of the Divide. In the 19th century, however, the three areas had identical mammalian species representation, although not equally dense populations. Fossils of an early Oligocene biota have been collected from the Douglass Creek basin. Presence of all but one of the Douglass Creek mammalian species in the Pipestone Springs-Canyon Ferry early Oligocene fauna suggests that the three ancient ecosystems resembled each other in much the same way as the 19th century systems. The early Oligocene deposits and biota of the Douglass Creek basin indicate a moist, temperate climate with seasonal variations. Sediment size and distribution suggest that the cross-valley relief was no greater than it is now. The fish and invertebrate faunas show that a shallow, hard-water lake existed in the area. The flora included a lowland, lake-border association and an upland coniferous forest. Although the ancient Douglass Creek biota doubtless included many species not represented in the fossil collections, most of the mammalian species are probably represented in the combined Douglass Creek, Pipestone Springs, and Canyon Ferry fossil assemblages. If so, the number of mammalian species was about the same as in the 19th century ecosystem.