The many episodes of mountain building and volcanism all left their lasting and unmistakable imprints across the face of the Yellowstone country. During the latter part of the Tertiary Period, erosion, too, had begun to make its own deep marks. But only in the last 100,000 years or so have the powerful exterior forces of the earth—chiefly running water and moving ice—had a virtually free hand in shaping the Park’s landscape. Nevertheless, in this short period of time they have wrought profound changes. Glaciation A giant boulder of Precambrian gneiss lies among the trees beside the road leading to Inspiration Point on the north rim of the Grand Canyon of the Yellowstone (fig. 34). This boulder, measuring approximately 24×20×18 feet and weighing at least 500 tons, is of considerable interest, not so much for its great size but because it is completely out-of-place in its present surroundings. The boulder rests on rhyolite lava flows of Quaternary age, at least 15 miles from the nearest outcrops of the ancient gneiss to the north and northeast. Obviously, this seemingly immovable chunk of rock was pushed or carried a long way by some very powerful transporting agent before it was finally dropped. A natural force of such magnitude could only have been exerted by moving ice; in fact, no further proof than this one boulder is needed for us to conclude beyond question that glaciers once existed in Yellowstone. There is, to be sure, much additional evidence that the Park region was extensively glaciated. Deposits of out-of-place boulders (_glacial erratics_), like the one mentioned above, are found nearly everywhere (fig. 35) and the mountains and high valleys still bear the vivid scars of ice sculpturing (figs. 36 and 37). [Illustration: GIANT BOULDER (glacial erratic) of Precambrian gneiss near Inspiration Point on the north rim of the Grand Canyon. The boulder, measuring 24×20×18 feet and weighing more than 500 tons, was dropped at this locality by glacial ice; it now rests on the much younger Plateau Rhyolite. The distance that the boulder was carried or pushed was at least 15 miles. (Fig. 34)] The principal requirement for the formation of glaciers is simple: more snow has to accumulate during the winter than is melted during the summer. If this condition continues for a long enough period of time (measured in centuries), the snow compacts to ice, and extensive icefields grow until they finally begin to move under their own weight, thereby becoming glaciers. Records show that the average year-round temperature is 32°-33°F along Yellowstone Lake, 35°F at Old Faithful, and 39°F at Mammoth. Each winter, snow accumulates to depths of 5-10 feet throughout much of the Park. If the average annual temperatures were to decrease a few degrees or the yearly snowfall were to increase a foot or so, either change could possibly herald the beginning of another ice age in the Yellowstone region.