Pollen and plant macrofossil records from Yellowstone National Park, if combined with dated glacial events, provide a paleoenvironmental record of much of the last glacial-interglacial cycle. Bull Lake glaciation has been dated at ∼140,000 yr B.P. Section EP-6 records a late-glacial to full interglacial sequence which is correlated with the Sangamonian interglacial and estimated to be 127,000 yr old at the peak warm period. A prevailing Pseudotsuga-Pinus flexilis-Pinus ponderosa forest suggests that climate was considerably warmer than any in the Holocene. Section EP-5 is somewhat younger, probably late Sangamonian, and shows a forest dominated by Picea, Abies, Pseudotsuga, and a haploxylon Pinus. Slightly cooler conditions than those of the present are indicated.
The warmest phase of the Grassy Lake Reservoir section is considered to be ∼82,000 yr old and records a warm interstadial cycle beginning with tundra. The warming sequence is indicated by change to a Picea-Abies-Pinus albicaulis forest and then to a Pinus contorta forest. The cycle ends with forest destruction and a return to open (tundra?) and, presumably, cold conditions.
Extremely low values of arboreal pollen indicate that tundra vegetation and cold conditions continued from ∼70,000 to ∼50,000 yr B.P. One cool and, apparently, short interstadial interrupted this extended period of tundra shortly after 68,000 B.P. when a very open parkland of Picea-Abies-Pinus albicaulis appeared.
No records of environment are available between ∼50,000 and 30,000 yr B.P. The Pinedale icecap apparently expanded ∼30,000 yr B.P. and lasted until ∼14,000 yr B.P. Late-glacial Picea-Abies-Pinus albicaulis parklands gave way to Pinus contorta forests that have prevailed with minor variation for ∼10,000 yr.