Fragmentary skeletal remains discovered in 1979 in southeastern Wyoming, associated with a mammalian fauna of early Hemingfordian age (∼18.2 to 18.8 Ma), represent the oldest known occurrence of dome-skulled chalicotheres in North America. The chalicothere and accompanying fauna were found in silica-cemented fluvial sandstones and conglomerate capping a series of west-to-east trending buttes in Goshen County, Wyoming, extending eastward into Sioux County, Nebraska. These butte caprocks, herein named the Carpenter Ranch Formation, are the reversed topographic remnants of a major paleovalley incised in the early Miocene and filled with coarse granitic/mafic gravels and sands derived from Precambrian-cored upifts to the west. The Carpenter Ranch Formation is identified as a coarser-grained, westward extension of the early Hemingfordian Runningwater paleovalley of Skinner, Skinner, and Gooris (1977) situated in northwest Nebraska. The age-diagnostic mammal faunas and coarse crystalline gravels of the Carpenter Ranch Formation of Wyoming and the coeval Martin Canyon beds of northern Colorado document the timing of tectonic rejuvenation of Rocky Mountain uplifts to the west in the early Hemingfordian.

A second line of buttes situated 3 miles (∼ 4.8 km) north of the Carpenter Ranch paleovalley includes the regional landmark recorded as “Spoon Butte” in 19th century military and topographic surveys of the region. Spoon Butte, its caprock in northwest-southeast alignment with adjacent satellite buttes, represents a second and younger mid-Miocene paleovalley trend, characterized by silica-cemented sandstones and basal granitic/chert gravel with Barstovian mammals.

N.H. Darton and G.I. Adams of the U.S. Geological Survey first described these caprocks as Arikaree conglomerates and mistakenly assumed they were of the same age as the caprock at Spoon Butte and its accompanying buttes to the north. However, this investigation shows that (a) mammal faunas from the two caprock trends are of markedly different ages; (b) the clast composition of the gravels is distinct; and (c) the paleovalleys align along two entirely different geographic trends.

Furthermore, a loose gravel found on top of Spoon Butte, previously believed to date the caprock, is identified as a lag deposit sharing the same distinctive clast composition as high Quaternary terrace gravels in the vicinity. These terrace gravels include acid volcanics (not present in the Carpenter Ranch and Spoon Butte basal gravels) and a composite assemblage of water-worn mammal bones of Barstovian, Clarendonian, and Pleistocene ages. Thus they demonstrate that late Cenozoic deposits once occupied the extensive tableland east of the Hartville Uplift, but were later removed by erosion and reworked into Pleistocene and Holocene drainages.