GLACIATED TERRAIN along the Northeast Entrance road. The boulders, many of them measuring 10 feet across or more, were carried into the area by ice flowing down Slough Creek from mountains north of the Park during the Pinedale Glaciation. As the glaciers melted, the boulders were left stranded in hummocky, morainal deposits. Shallow depressions in the irregular topography are now commonly filled by small ponds. (Fig. 35)
For the next 10,000 years, the ice thickened and spread out over more and more of the Park area. The mass centered over the Yellowstone Lake basin grew to a depth of 3,000 feet or more and dominated the entire scene; it formed a broad “mountain” of ice which became so high that it caused more snow to fall upon itself and was cold enough to prevent much of this snow from melting. Eventually the Pinedale glaciers covered about 90 percent of Yellowstone ([fig. 38]).
CANYON PROFILES. Typical profiles of a canyon cut by a stream (A) and of a canyon gouged by a glacier (B). Glacial cirques (C) are shown at the head and high on the side of the glaciated valley. (Fig. 36)
GLACIAL CIRQUE on east face of Electric Peak, northern Gallatin Range. During several episodes of glaciation, this steep-walled amphitheaterlike valley was cut and filled by ice which fed glaciers moving downslope to the lower right. The cirque floor is now covered by a thick deposit of rock rubble underlain in part by ice, and the whole mass is still moving slowly downhill as a rock glacier. The dark rock at lower right is part of the Electric Peak stock, composed of diorite ([fig. 20]) and other kinds of intrusive igneous rocks. The rocks in the cirque walls are chiefly Cretaceous shales (light to moderately dark color) with thin sills of igneous rock (very dark color). (West-looking oblique aerial photograph, courtesy of William B. Hall, University of Idaho.) (Fig. 37)
After their maximum advance, the Pinedale glaciers began to melt, leaving behind the rock debris they had gouged from the landscape and had pushed or carried along with them. These glacial moraines are now found in many areas throughout the Park. In places, glacial ice and (or) rock debris formed natural dams across stream valleys, thereby impounding lakes. Parts of Hayden Valley, for example, contain layers of very fine sand, silt, and clay several tens of feet thick ([fig. 39]) that accumulated along the bottom of a large lake. This lake formed behind a glacial dam across the Yellowstone River near Upper Falls. Some of the glacial dams broke and released water catastrophically, causing giant floods; the occurrence of one such flood is particularly evident along the Yellowstone River valley near Gardiner, Montana.
By about 12,000 years ago the thick Pinedale ice sheet had melted entirely from the Yellowstone Lake basin and most other areas of the Park, although valley glaciers continued to exist in the mountains until about 8,500 years ago. Then, following a short period of total disappearance, small icefields formed again in the heads of some of the higher mountain valleys. Since the melting of the Pinedale ice, however, none has descended as a glacier into the lower stretches of the valleys. Even though a few snowfields persist locally throughout the summers (except during the warmest years), no glaciers exist in the Park at the present time.
EXTENT OF ICE in Yellowstone National Park during the maximum spreading of the Pinedale glaciers, probably about 15,000 years ago. Long arrows indicate direction of strong flowage of ice; short arrows show direction of less vigorous ice flowage. The dark-blue area shows the main ice mass centered over the Yellowstone Lake basin in the southeast corner of the Park. Many of the high peaks and ridges such as Mount Washburn, which are here shown free of ice, were glaciated at least once during the past 250,000 years. Whether they were covered by the Pinedale glaciers, however, is still an unresolved question. (Based on information supplied by G. M. Richmond, K. L. Pierce, and H. A. Waldrop.) (Fig. 38)