The wood rats of the Pit fauna of Porcupine Cave (2900 m elevation) include three species (Neotoma mexicana, N. micropus, N. floridana) that now live in the vicinity of Pueblo, Colorado (1420 m elevation) and in southeastern Colorado in the western part of the Great Plains. A fourth (N. stephensi) now lives farther southwest than Colorado, and a fifth (N. cinerea) is characteristic of elevations above 1800 m in the Rocky Mountain region. The wood rats of the DMNH Velvet Room excavation have not yet been examined in stratigraphic context, but in general they seem to more closely resemble the modern fauna in yielding N. cinerea with very rare exceptions.

The cave itself is on a ridge of Paleozoic rock protruding above a beveled surface that truncates older rocks and appears to represent a preserved part of the Tertiary Rocky Mountain Peneplain (Lee, 1923). Faulting before 780 Ka ago elevated the ridge above the plain to make the cave available to the wood rats.

The wood rat fauna of the Pit locality indicates that what is now considered Great Plains habitat was continuous with South Park prior to and possibly around 780 Ka ago. This situation would be consistent with an elevation of the cave no higher than 1830 m at the time the fossil deposits accumulated. This is the lowest modern elevation of Neotoma cinerea and the approximate highest modern elevation of the other wood rats of the Pit fauna where they occur around Pueblo. The wood rat assemblage would also be consistent with differing glacial climatic patterns in the "Nebraskan" as compared to the "Kansan" and later glacial advances.

At perhaps the same time as the uppermost Pit levels were deposited, or more probably slightly more recent in time than the Pit sequence, the Bishop Ash was deposited in silts, sands, and gravels in the Upper Arkansas Valley west of Porcupine Cave. The Bishop Ash was also deposited at the same time that the "Nebraskan" glacial advances E, F, and G of Richmond (1986), as correlated to Shackleton and Opdyke (1976), flowed across the northeastern part of the Great Plains. At this time and presumably during earlier "Nebraskan" glaciations, the

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