The Pitchfork formation is here defined as the lowest series of beds in the southeastern part of the Absaroka Range, Wyoming. The Pitchfork formation consists largely of detrital rocks—sandstones, siltstones, and conglomerates—containing volcanic débris from eruptive centers in the Yellowstone Park-Absaroka volcanic province. The thickness of the Pitchfork formation ranges from about 200 to about 1,200 feet and is generally about 1,000 feet. The formation interfingers northward with the early basic breccia of Hague (1896 and 1899); toward the south it pinches out against the axis of the Owl Creek Range, forming the southern margin of the Bighorn basin. An estimated 80 per cent of the detrital beds are characterized by andesitic débris; dacitic detritus predominates in most of the remainder. Most beds contain a small proportion of non-volcanic débris, and a few beds contain entirely non-volcanic material. Several varieties of tuff occur in this formation. Most of the detrital material was deposited in stream channels and fioodplains, in a warm, humid climate. Both vertebrate and plant remains indicate that the Pitchfork formation is probably middle Eocene. The Pitchfork formation is almost everywhere underlain by the Willwood and Tatman formations of the Bighorn basin; it is overlain by the early basalt sheets of Hague (1896 and 1899) and equivalent breccia and by conglomerates of the late basic breccia of Hague (1896 and 1899).
Several regional correlations of the Pitchfork formation and underlying beds are suggested. The Pitchfork formation intertongues with and is equivalent to the early basic breccia of Hague (1896 and 1899). Relationship of the Pitchfork formation to the early acid breccia of Yellowstone Park is uncertain; the early acid breccia may be older than the Pitchfork formation, but part of the early acid breccia may be equivalent to beds of the early basic breccia; hence, to part of the Pitchfork formation. Only traces of volcanic material are present in the Willwood and Tatman formations of the Bighorn basin, and much of this volcanic material is altered and is probably Paleocene or older. Unaltered dacitic material is locally abundant in the Wind River formation of the Wind River basin. Distribution and lithologic character of the volcanic material in the Wind River formation suggest that it is a product of early Eocene eruptions probably representing the early acid breccia in the southern part of the Absaroka Range. The absence of similar volcanic material in equivalent beds of the Bighorn basin is explained by prevaiUng winds from the northwest or north. The Aycross formation, at the southern margin of the Absaroka Range, is probably correlative with the Pitchfork formation and the early basic breccia.