On the Jackson Hole side the Teton Range presents one of the most precipitous mountain fronts on the continent. Except for Teton Pass, at its southern end, the range is practically an insuperable barrier. Forty miles in length, it springs abruptly from Jackson Hole and only a few miles west of its base attains elevations of more than 13,000 feet above the sea. Thus most of the range is lifted above timber line into the realm of perpetual snow, and in its deeper recesses small glaciers still linger. The grandeur of the beetling gray crags, sheer precipices, and perennial snow fields, is vastly enhanced on this side by the total absence of foothills and by contrast with the relatively flat floor of Jackson Hole, from which they are usually viewed.

The Teton Range may be described as a long block of the earth that has been broken and uplifted along its eastern margin, thus being tilted westward. Movement of this sort along a fracture is what the geologist terms "faulting." The total amount of uplift along the eastern edge of the block amounts to more than 10,000 feet. Doubtless this uplift was accomplished not by one cataclysm but by a series of small faulting movements distributed over a very long period. Probably the time of faulting was as remote as the middle of the Tertiary period (the period just before the Ice Age, the latest chapter of the earth's history).

Very impressive is the contrast between the east and west sides of the Teton Range. From the east, the Jackson Hole side, one views the precipitous side of the mountain block as it has been exposed by uplift and erosion. From the west, the Idaho side, is seen the broad top of the block, which is gently inclined toward the west. In the eastern front, furthermore, one sees the ancient, deep-seated crystalline rocks (gneiss, schist, and pegmatite) belonging to the earliest known geologic eras, the pre-Cambrian. In places on the top of the block, at the head of Death and Avalanche Canyons, for example, are seen the inclined layers of limestone, quartzite, and shale belonging to the less ancient Paleozoic era. These layers formerly covered the entire block, but they have been worn away from half of the area, thus exposing the underlying crystallines. The west and north flanks of the range are overlapped by relatively young beds of lava that are continuous with those covering eastern Idaho and the Yellowstone plateaus.

JACKSON HOLE

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.