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stages, during which extensive ice fields formed, and interglacial stages, during which these were largely or wholly withdrawn.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.

With two exceptions each of the large moraines incloses a lake. In this way Phelps, Taggart, Bradley, Jenny, Leigh, and Jackson Lakes originated; all ranged along the western border of Jackson Hole. No lakes were formed along the eastern border, inasmuch as on this side no glaciers extended beyond their canyons. String Lake is dammed in part by a gravel fill.

Outwash plains are the deposits formed by streams which, during the Ice Age, issued from the glaciers. Of this origin are the broad, cobble-strewn flats, usually overgrown with sage, which cover the floor of Jackson Hole. They are diversified by bars, abandoned stream channels, terraces and "pitted plains", features of exceptional interest to one who examines them in detail. Several isolated buttes—Signal, Blacktail, and the Gros Ventre Buttes—rise like islands a thousand feet or more above these flats.

Each canyon gives evidence of the vigor with which the glacier it once contained gouged out its channel. In many places the rock of the broad floors and steep sides is still remarkably polished. Every canyon leads up to one or more amphitheaters, or cirques, with sheer bare walls hundreds of feet high. Tracing these ice-gouged canyons headward one will discover many rock-rimmed lakelets, some hung on precipitous mountain sides where one might be pardoned for asserting that no lake could possibly exist.