That masterpiece was begun by water, accomplished by ice, and finished by water. In the beginning, streams determined the direction of the valleys and carved these valleys deep. Then came, in very recent times, as geologists measure earth's history, the Great Ice Age. As a result of falling temperature, the mountains became covered, except their higher summits and the continental divide, with glaciers. These came in at least two invasions, and remained many hundreds of thousands of years. When changing climate melted them away, the Rocky Mountain National Park remained not greatly different from what it is to-day. Frosts and rains have softened and beautified it since. These glaciers, first forming in the beds of streams by the accumulations of snow which presently turned to ice and moved slowly down the valleys, began at once to pluck out blocks of granite from their starting-points, and settle themselves in cirques. They plucked downward and backward, undermining their cirque walls until falling granite left precipices; armed with imprisoned rocks, they gouged and scraped their beds, and these processes, constantly repeated for thousands of centuries, produced the mountain forms, the giant gorges, the enormous precipices, and the rounded granite valleys of the stupendous east elevation of the Front Range. There is a good illustration in Iceberg Lake, near the base of Trail Ridge on the Ute Trail. This precipitous well, which every visitor to Rocky Mountain should see, originally was an ice-filled hollow in the high surface of the ridge. When the Fall River Glacier moved eastward, the ice in the hollow slipped down to join it, and by that very motion became itself a glacier. Downward and backward plucking in the cirque which it presently made, and the falling of the undermined walls, produced in, say, a few hundred thousand years this striking well, upon whose lake's surface visitors of to-day will find cakes of floating ice, broken from the sloping snow-field which is the old glacier's remainder and representative of to-day. The glaciers which shaped Rocky Mountain's big canyons had enormous size and thickness. Ice streams from scores of glacial cirques joined fan-like to form the Wild Basin Glacier, which swept out through the narrow valley of St. Vrain. Four glaciers headed at Longs Peak, one west of Mount Meeker, which gave into the Wild Basin; one west of Longs Peak, which joined the combination of glaciers that hollowed Loch Vale; one upon the north, which moulded Glacier Gorge; and the small but powerful glacier which hollowed the great Chasm on the east front of Longs Peak. The Loch Vale and Glacier Gorge glaciers joined with giant ice streams as far north as Tyndall Gorge to form the Bartholf Glacier; and north of that the mighty Thompson Glacier drained the divide to the head of Forest Canyon, while the Fall River Glacier drained the Mummy Range south of Hagues Peak.