• The stratigraphic section consists of

•fluvial sequences of massive to crossbedded sandstone with conglomeratic interbeds, ranging in thickness from a few centimeters to a meter (Fig. 3). The sequence is capped by a silt- and clay-filled channel cutting into a thick, resistant, bedded sandstone. Fossil material is derived from two layers within the massively bedded sand- stone. The lowermost of these two layers has been dubbed the “bone-hash” layer, as it consists of a pebble-sized con- glomerate stringer with numerous bone fragments of bony fish, horses and other vertebrates. As of the 2007 season, the “bone hash” layer has been covered by at least 1.5 m of newly slumped material, but the loose nature of this slumped material does not preclude digging out this extremely productive horizon. The second productive layer is referred to as the “rodent skull” layer because one of the first finds was an extremely fragmented, edentulous (teeth missing) mylagaulid rodent skull. The “rodent skull” layer is a massive, silty sandstone with numerous, pebble-sized white concretions. These concretions may have developed as a result of subaerial exposure before being covered by another fluvial sequence.