The reasons for the climatic changes that caused the Ice Age are still a matter of much scientific debate. Various theories have been advanced that attribute them to changes in solar radiation, changes in the earth’s orbit and inclination to the sun, variations in the amount of carbon dioxide in the atmosphere, shifts in the positions of the continents or the poles, and to many other factors, but none has met with universal acceptance. No doubt the explanation lies in some unusual combination of circumstances, for widespread glaciation occurred only twice before in the earth’s history—once in the late Precambrian and once during the Permian. It is quite clear, however, that the glaciers did not form in response to any local cause such as the uplift of the Teton Range, for concurrent climatic changes and ice advance took place throughout many parts of the world. At least three times in the last 250,000 years glaciers from the surrounding highlands invaded Jackson Hole. The oldest and most widespread glaciation probably took place about 200,000 years ago; it was called the _Buffalo Glaciation_ by Prof. Eliot Blackwelder in 1915 (see selected references). The age estimate is based on measurements of the thickness of the decomposed layer on the surface of obsidian pebbles in the glacial debris. Major sources of ice were the Beartooth Mountains (fig. 1), the Absaroka Range, and the Wind River Range. The Gros Ventre Mountains and Teton Range furnished lesser amounts of ice. The ice from the Beartooth and Absaroka centers of ice accumulation converged in the northeastern part of Grand Teton National Park and flowed south along the face of the Teton Range in a giant stream that in many places was 2,000 feet thick (fig. 57). All but the highest parts of the Pinyon Peak and Mount Leidy Highlands were buried and scoured. Signal Mountain, Blacktail Butte, and the Gros Ventres Buttes were overridden and shaped by ice at this time. Another glacier, this one from the Wind River Range, flowed northwest along the Continental Divide, then down the Gros Ventre River Valley, and merged with the southward-moving main ice stream west of Lower Slide Lake. Where Jackson Hole narrows southward, the glacier became more and more confined, but nevertheless flowed all the way through the Snake River Canyon and on into Idaho. [Illustration: Figure 58. _Glacial deposits, outwash, and loess exposed along Boyle Ditch in Jackson Hole National Elk Refuge. Indicated are middle Pliocene Teewinot formation (A), oldest till (B), Bull Lake outwash gravel (C), and post-Bull Lake loess (D), which here contains snail shells dated by Carbon-14 as 15,000 years old. Height of cliff is about 30 feet._]