<P> Periods of glaciation occurred from the Pleistocene Epoch (1.8 million--70,000 years ago) to the Holocene Epoch (fewer than 11,000 years ago). These ice ages left their mark on the Rockies, forming extensive glacial landforms, such as U-shaped valleys and cirques . Recent glacial episodes included the Bull Lake Glaciation, which began about 150,000 years ago, and the Pinedale Glaciation, which perhaps remained at full glaciation until 15,000--20,000 years ago . </P> <P> All of these geological processes exposed a complex set of rocks at the surface . For example, volcanic rock from the Paleogene and Neogene periods (66 million--2.6 million years ago) occurs in the San Juan Mountains and in other areas . Millennia of severe erosion in the Wyoming Basin transformed intermountain basins into a relatively flat terrain . The Tetons and other north - central ranges contain folded and faulted rocks of Paleozoic and Mesozoic age draped above cores of Proterozoic and Archean igneous and metamorphic rocks ranging in age from 1.2 billion (e.g., Tetons) to more than 3.3 billion years (Beartooth Mountains). </P> <P> There are a wide range of environmental factors in the Rocky Mountains . The Rockies range in latitude between the Liard River in British Columbia (at 59 ° N) and the Rio Grande in New Mexico (at 35 ° N). Prairie occurs at or below 1,800 feet (550 m), while the highest peak in the range is Mount Elbert at 14,440 feet (4,400 m). Precipitation ranges from 10 inches (250 mm) per year in the southern valleys to 60 inches (1,500 mm) per year locally in the northern peaks . Average January temperatures can range from 20 ° F (− 7 ° C) in Prince George, British Columbia, to 43 ° F (6 ° C) in Trinidad, Colorado . Therefore, there is not a single monolithic ecosystem for the entire Rocky Mountain Range . </P> <P> Instead, ecologists divide the Rocky Mountain into a number of biotic zones . Each zone is defined by whether it can support trees and the presence of one or more indicator species . Two zones that do not support trees are the Plains and the Alpine tundra . The Great Plains lie to the east of the Rockies and is characterized by prairie grasses (below roughly 1,800 feet (550 m)). Alpine tundra occurs in regions above the treeline for the Rocky Mountains, which varies from 12,000 feet (3,700 m) in New Mexico to 2,500 feet (760 m) at the northern end of the Rocky Mountains (near the Yukon). </P>

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