Great Basin Naturalist


Five alcoves (rock shelters) in the Forty-Mile Canyon–Willow Gulch area of the Escalante River Basin in southeastern Utah yielded rich deposits of late Quaternary macrobotanical remains. The deposits were sampled and the contents identified in order to construct a chronology of vegetational change. Fourteen radiocarbon dates indicate that the fossils were deposited between 12,690 and 7510 yr B.P. (years before present).

Ninety-one plant taxa were identified, 62 to species. Six species were common to all alcoves: Gambel oak (Quercus gambelii), box-elder (Acer negundo), prickly pear (Opuntia subgenus Platyopuntia), skunkbush (Rhus aromatica var. trilobata), serviceberry (Amelanchier utahensis), and Indian ricegrass (Oryzopsis hymenoides).

Late Pleistocene samples (>11,000 yr B.P.) contain extralocal, elevationally depressed species such as Douglas fir (Pseudotsuga menziesii), spruce (Picea sp.), and mountain mahogany (Cercocarpus ledifolius), and mesophytic species such as rose (Rosa woodsii) and water birch (Betula occidentalis). Early Holocene samples (11,000–8000 yr B.P.) contain no elevationally depressed conifers, and the remaining mesophytic species decrease in relative abundance. Reticulated hackberry (Celtis reticulata) becomes common. The terminal Early Holocene sample (8000–7000 yr B.P.) contains abundant Gambel oak and prickly pear, but little else.

Paleoclimatic interpretations for the Late Pleistocene correspond well to those of most other workers on the Colorado Plateau. Climates that were wetter and at least seasonally cooler than they are today are inferred from the macrobotanical assemblage. However, the increased moisture is attributed to higher stream base levels and increased groundwater rather than directly to increased precipitation. Early Holocene climates are interpreted as warmer and drier than those of the Late Pleistocene but still wetter than the present climate. Groundwater levels appear to be decreasing due to stream entrenchment. Terminal Early Holocene climates were much warmer and at least seasonally drier. By the end of the period, groundwater levels had decreased so much that the alcoves were unable to sustain plant communities; stream base level was probably near the present level.