Jackson Hole, which adjoins the park on the southeast, is one of the most sequestered valleys in the Rockies, encompassed on all sides as it is by mountain barriers. It is 48 miles long, for the most part 6 to 8 miles wide, and embraces an area of more than 400 square miles. The floor of the valley slopes from an altitude of 7,000 feet at the north end to 6,000 at the south. Jackson Hole lies a few miles west of the Continental Divide, and occupies the central portion of the headwaters area of the Snake River. Mountain streams converge radially toward it from the surrounding highlands, and the Snake River receives these as it flows through the valley. Jackson Hole has largely been excavated by the Snake River and its tributaries from the shale formations which once extended over the region to a depth of several thousand feet. Rocks surrounding the region, being more resistant, were reduced less rapidly and therefore have been left standing in relief as highlands. THE WORK OF GLACIERS Here, as in several other national parks, the glaciers of the Ice Age, known to the geologist as the Pleistocene period, played a leading role in developing the extraordinary scenic features. Just as the streams now converge toward Jackson Hole, so in ages past glaciers moved down toward, and in many instances into, the basin from the highlands to the east, north, and west. Detailed study has shown that the Ice Age was not a single, simple episode, but is divisible into "stages"--glacial stages, during which extensive ice fields formed, and interglacial stages, during which these were largely or wholly withdrawn. The duration of each is to be thought of in terms of tens of thousands of years. In Jackson Hole, three glacial and two interglacial stages have been recognized. Only the most recent glacial stage need concern us here, the other two having occurred so long ago that their records are much obscured. This stage ended but yesterday, geologically speaking, and to it is due much of the grandeur of the region. In the Teton Range every canyon from Phillips northward contained a glacier, and many of these reached eastward to the base of the range where they spread widely upon the floor of Jackson Hole. Where Jackson Lake now is there lay a great, sluggish field of ice resulting from the confluence of adjacent alpine glaciers. Moraines, outwash plains, and lakes are easily recognizable features that originated during the latest glacial stages, and most of the peaks and canyons were greatly modified. Moraines are deposits of debris, piled up by the ice itself. Such are the heavily wooded, hummocky embankments which rest along the base of the mountains from Granite Canyon northward, rising in some cases 200 or 300 feet above the floor of Jackson Hole and heaped with enormous boulders quarried by the ice far back in the range.