GIacial times never seem far away in the Greater Yellowstone Ecosystem. If you were to walk the streets of Chicago on a hot August day, you would have a rough time believing that 150 centuries ago the land beneath the sidewalk was covered by ice thousands of feet thick. In Grand Teton National Park, on the same August day, you can view a glacier. From the valley floor, the Teton Glacier is only 4.5 miles to the west and 7,000 feet up the mountain. More glaciers grace the flanks of nearby Mt. Moran.

On the Pitchstone Plateau of Yellow-stone, a hiker in August can find patches of last winter's snow in the shade of obsidian ledges, if they aren't already covered by the new snows of autumn. The mean annual temperature at Lake Ranger Station in Yellowstone is 33° F, just a tad above freezing. Possibly the most striking examples of Yellowstone's deep chill are Yellowstone and Lewis lakes that remain frozen most years from December to late May. Often Lewis Lake melts in early June.

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Winter weather in northwest Wyoming is brisk. Temperatures around -40° F routinely occur at Old Faithful, and those in the town of Jackson, Wyoming, are often below -30° E With these temperatures in mind, it is not difficult to imagine the effect colder, more cloudy summers and temperature drops of perhaps 15° F might have on the Greater Yellowstone Ecosystem.

In this chapter we will look in detail at Yellowstone's youngest glaciation, the Pinedale, and make some general observa tions about its predecessor, the Bull Lake Though there were eight or more earlier glaciations than Bull Lake in the Yellowstone region, we know little about them. We will follow the growth of the Pinedale glaciers in Yellowstone and Grand Teton national parks and their immediate environs. We will look at land-forms and deposits the glaciers left behind, and try to explain how these landforms and deposits were formed. !

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. Sagebrush and tundra grasses hug the treeless slopes and foothills around you; dwarf willows border restless streams. Far to the south-west, you see clumps of trees in protected areas.

If you stood here at the Snake River Overlook on a stormy September day, you might have trouble standing at all. The cool summer breeze is now a blustery chill blast. The muddy streams of summer have shrunk to trickles-many, in fact, are empty, their dried-out channels caked with silt and mud. The fierce winds of autumn whip the dry sediments into rolling clouds which, carried south, fall as blankets of windblown dust called loess. Today, on high terraces south of Jackson, deposited during the last glaciation, loess is as much as 20 feet thick.

Now back to summer and a major adventure: We decide to explore the huge ice field to the north. We pick a route up its sloping face and begin the difficult climb up the glacier's back. At first the slope is steep (more than 500 feet/mile), but soon it flattens to less than 100 feet/mile then becomes nearly horizontal.

To the east a vast sea of featureless ice is punctuated by the isolated peaks of Whetstone and Gravel mountains. To the west stands the Teton Range high above the Snake River valley. The ice sheet beneath us extends westward nearly to the base of the Tetons. The Snake River valley to the north is ice-free for miles, but at its northern end is a lobe of Yellowstone ice filling it from wall to wall. Between the lobe of Yellowstone ice advancing down the Snake River Valley and the lobe we are standing on is an ancient Jackson Lake whose gray-green waters are dotted with icebergs calved from valley glaciers pouring out of the Tetons.

As we continue our trek, and as the air chills, we notice that runoff on the glacier's surface is becoming less and less. At about 9,000 feet elevation, the bare ice gives way to slush, then old dry snow. We have reached snowline on the glacier. Up, up, day after day, we finally cross the southern boundary of Yellowstone. Behind us are the Teton peaks. Ahead on our left the summit of Mt. Sheridan lies a few hundred feet beneath the ice. On our right, a chain of dark knobs barely piercing an expanse of white marks the crest-line of the Absaroka Range.

As we approach the vicinity of present-day Yellowstone Lake, the ice underfoot is about 4,000 feet thick. In every direction, to the very horizons, a boundless, unrelieved plain of snow-covered ice lies silent and lifeless under a glaring sun. We have reached the summit of the Yellowstone ice field.

On the flat, nearly featureless icescape we follow a compass course north. We pause above the buried crest of the Washbum range reflected by broad, subtle mounds in the nearly horizontal surface of the ice. Far to the northeast, a mighty dome of snow and ice mantles the granite massif of the Beartooth Mountains. To the northwest are the nearby peaks of the Gallatins and the faraway peaks of the Madison Range. To the south bulks the broad summit dome of the ice field.

We cross ice-buried hot springs, progenitors of the Mammoth Terraces, about three thousand feet beneath our boots. Just beyond to the north, the gentle slope of the Yellowstone valley glacier steepens and curves below Yankee Jim Canyon . Eventually we make our way through the crevasses down to the terminal ice face near Chico Hot Springs in Montana. Our traverse from the Snake River Overlook across the buried Washbum Range has taken us across an ice divide at an altitude of about 11,500 feet. We have covered a distance of about 120 miles in a 10-day journey on trackless ice. From Chico Hot Springs, could we have seen the southern margin of the great continental ice sheet to the north? No, we would have had to trudge another 150 miles north across windy, cold, sparsely vegetated plains to the vicinity of Great Falls, Montana, to reach the edge of that ice mass.