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.