used by Native Americans starting thousands of years ago to make arrowheads, knives, and spear points. The lake was dry by the time a series of enormous pyroclastic flows from the Yellowstone area buried Jackson Hole under welded tuff. Older exposures of this tuff are exposed in the Bivouac Formation at Signal Mountain and Pleistocene-aged tuffs are found capping East and West Gros Venture Buttes (both the mountain and buttes are small fault blocks). Climatic conditions in the area gradually changed through the Cenozoic as plate tectonics moved North America northwest from a sub-tropical to a temperate zone by the Pliocene epoch. The onset of a series of glaciations in the Pleistocene epoch saw the introduction of large glaciers in the Teton and surrounding ranges, which flowed all the way to Jackson Hole during at least three ice ages. Cascade, Garnet, Death and Granite Canyons were all carved by successive periods of glaciation. The first and most severe of the known glacial advances in the area was caused by the Buffalo glaciation. In that event the individual alpine (mountain valley) glaciers from the Tetons' east side coalesced to form a 2,000 foot (610 m) thick apron of ice that overrode and abraded Signal Mountain and the other three buttes at the south end of Jackson Hole. Similar dramas were repeated on other ranges in the region, eventually forming part of the Canadian Ice Sheet, which at its maximum, extended into eastern Idaho. This continental-sized glacial system stripped all the soil and vegetation from countless valleys and many basins, leaving them a wasteland of bedrock strewn with boulders after the glaciers finally retreated. Parts of Jackson Hole that were not touched by the following milder glaciations still cannot support anything but the hardiest plants (smaller glaciers deposit glacial till and small rocks relatively near their source, while continental glaciers transport all but the largest fragments far away). A less severe glaciation, known as Bull Lake, started sometime between 160 and 130 thousand years ago. Bull Lake helped repair some of the damage of the Buffalo event by forming smaller glaciers which deposited loose material over the bedrock. In that event, the large glacier which ran down Jackson Hole only extended just south of where Jackson, Wyoming, now sits and melted about 100,000 years ago. Then from 25,000 to 10,000 years ago the lower volume Wisconsin glaciation carved many of the glacial