Paleo Slide Set: Packrat Middens: Vegetation & Climate Variability in the Southwestern United States
Selected taxa from midden macrofossils, Allen Canyon Cave, San Juan County, Utah.

The relative abundance of key species throughout time at Allen Canyon Cave, organized from left to right, 
reflect Pleistocene epoch departures and Holocene arrivals. At 11,300 14C yr. BP, a boreal forest of 
Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and limber pine (Pinus flexilis) 
occupied the site. These species currently grow 800 m higher in the Abajo Mountains. This boreal forest 
changed to a mixed-conifer forest, dominated by Douglas fir (Pseudotsuga menziesii) at ca. 11,000 14C yr. BP, 
coinciding with the timing of a major vegetation change across the southwestern U.S. Midden assemblages younger 
than 10,000 14C yr. BP record the sequential arrival of ponderosa pine (Pinus ponderosa), Colorado pinon, and 
Utah juniper. 

Photo Credits:
Julio Betancourt 
U. S. Geological Survey