During the last ice age, glaciers formed high in the Rocky Mountains and carved out the peaks and valleys visible today. Recreating the landscape and life forms of this era of the last great glaciations (from 10,000 to 125,000 years ago), this guidebook describes a little-known yet pivotal period in the ecological history of four western national parks: Glacier, Yellowstone, Grand Teton, and Rocky Mountain. Scott A. Elias describes how great sheets of ice spread over and changed the shape of the land - forming the steep-walled valleys and braided rivers of Glacier National Park, the chain of so-called "pater noster" lakes in the lower Rockies, and the end moraines that dammed Jenny, Bradley, Taggart, and Phelps lakes in the Grand Teton park. Drawing on fossil evidence, he also introduces the large animals that thrived 21,000 years ago - dire wolves, short-faced bears, American cheetahs, and mammoths - and that quickly died off at the end of the last glaciation. He recounts the coming of humans to the region, the ascendance of the ecosystems we see today, and the lasting features (plant, animal and topographical) of the ice age. This guidebook, along with its companion on the ice-age history of Alaskan national parks, relates as well the kinds of evidence and methods scientists use to recover past environments. Covering geology, climate, ancient plant and animal life, and human presence, Elias introduces paleoecology - the interactions among plants, animals, and the prehistoric ecosystem - to hikers, tourists, and armchair travelers.