A study of the fossils of the Madera Formation [Pennsylvanian], Colorado, has disclosed a sequence of fusulinids which help to outline the zones of Fusulinella and Fusulina in the Park area. The fossil collections also contain several other forms, hitherto not reported from this area, which have fairly narrow stratigraphic ranges. These include Erisocrinus typus and Ulocrinus sp. which indicate Des Moinesian or younger rocks and Schistocrinus ovalis, Stereostylus cf. S. aages, Lophamplexus brevifolius and Dibunophyllum moorei which indicate strata of Des Moinesian age. The Madera and associated Paleozoic formations in this area were deposited in a narrow trough that extended over the Park area from SE. to NW. Rapid stratigraphic and geographic variation from marine limestone to arkose and siltstone bearing plant fossils is indicative of the rapidly changing environment in which the Madera Formation was deposited. The more typical marine rocks, almost always silty or arkosic, vary from limestone and shale deposited in a restricted environment, to fragmental and oolitic limestones which accumulated above wave base, to strata deposited in quiet, more normal marine waters. Many arkose and siltstone beds bear terrestrial plant remains, however, quite a few are very calcareous and contain marine fossils. The Madera Formation was deposited during the transgressive phase of upper Paleozoic marine inundation in the area. At this time the depositional environments fluctuated between marine and nonmarine conditions. The Red Wing Formation which underlies the Madera Formation, and the Pass Creek and Sangre de Christo formations which overlie the Madera Formation are essentially nonmarine rocks. Respectively, they probably represent deposition during a pre-transgressive and a regressive phase of marine conditions.