But before we look at the details, try to imagine what Yellowstone and Grand Teton national parks looked like on a summer day 25,000 years ago at the height of Pinedale glaciation. The Yellowstone ice field at that time was near its maximum size. Imagine you are standing at the Snake River Overlook in Grand Teton, facing north. Immediately in front of you is a wall of ice that extends from the eastern valley wall nearly to the Tetons. Streams of muddy water pour down the glacier's blue face and flow from tunnels at its base.
Occasional boulders tumble from the ice face amid a constant cascade of gravel.
Muddy meltwater, gravel, and boulders enter a wide, shallow river heading two miles east near the present-day Lost Creek Ranch, between Shadow Mountain and the glacier before you. Braided channels of this river move constantly across a treeless plain, shifting from south to west and back again as they construct a huge alluvial fan of glacial debris. Although it is mid-summer, a cool breeze drifts down the glacier's face. To the west, the ice-sheathed peaks of the Teton Range tower above great rivers of ice filling the canyons and spilling out onto the plain. Far to the south-west, you see clumps of trees in protected areas.