A stratigraphically controlled succession of vertebrate faunas, presented as a composite section with regional geologic markers, is presented in figure 4. Within that framework, some further partitioning is possible based on preliminary evidence from the rodents and the new arvicoline rodent zonation for the central Great Plains provided by Martin (2003). For instance, Fox Canyon time is characterized by two geomyids ( Geomys minor [= G. smithi Hibbard, 1967] and the diminutive G. adamsi ), two arvicolids ( Pliophenacomys finneyi and Ogmodontomys poaphagus ), and the absence of cotton rats. Geomys adamsi is not encountered in the Meade Basin after Fox Canyon. It is a primitive species that has been tentatively identified only at the early Blancan Pipe Creek l.f. from Indiana (Martin et al ., 2002a). Likewise, P. finneyi is restricted to Fox Canyon. A large but morphologically primitive cotton rat, Sigmodon holocuspis , at KC and Raptor 1C is followed by a small, dentally archaic form of S . minor at Ripley B. Cotton rats are absent from the Wiens, Vasquez/Newt and Raptor 3-5 l.fs. We tentatively interpret the oscillations in cotton rat populations as evidence of climatic change, with times of absence implying colder winters (Martin, 1986; Martin and Fairbanks, 1999).
Local faunas recovered beneath the Bishop gravel south of the Cimarron River in Seward County, such as Hibbard's original Saw Rock Canyon (SRC) l.f. (Hibbard 1953; 1964) and our SRC 1, Argonaut, and Fallen Angel B l.fs. (Martin et al ., 2000), contain the archaic arvicolid Ogmodontomys sawrockensis Hibbard (1957). A very primitive cotton rat, identified only as Sigmodon sp., is present at Argonaut and Fallen Angel, but is absent from the lower part of the section with SRC and SRC 1. Once again, this probably indicates a climatic shift to either cooler or drier conditions or a combination of both.
Our field work over the past few years indicates that the stratigraphy of the Meade Basin is more complex than the hypotheses presented in older works ( e.g ., Hibbard, 1958; Zakrzewski, 1975; Bayne, 1976). The identity of some sediments referred to the Ballard Formation has been questioned (Martin et al ., 2002b), and we do not currently recognize this formation except at the type locality on the Big Springs Ranch, which is not accessible to scientists. It is also problematic to extend use of the Rexroad Formation south from its type locality near Meade State Park to the north bank of the Cimarron River, and until some thorny problems are resolved in the type area, we refrain from doing so. There are a number of Blancan gravel units in Meade County, at different stratigraphic levels, that have been miscorrelated in the past; we therefore choose to introduce the informal Wolf and Bishop gravels for those in southern Meade County simply to avoid confusion. Fortunately, in the Cimarron River area the Bishop and Wolf gravels and a series of caliche units (CC1, CC2, CC3) can be followed through the canyons and used effectively as stratigraphic markers.