[Illustration: A line of hikers approaches Arapaho Glacier, west of Boulder. Movement of the glacier is evidenced by the crevasses apparent just below the snowfield in the dirty gray glacial ice. (H. H. Heuston photo)] Several small glaciers are still present in the Colorado mountains, all in sheltered cirques above 11,000 feet. These may be remnants of the former larger glaciers, or new glaciers formed after a long warming episode. A hike to one of these glaciers is a rewarding experience for anyone interested in geology. Some of the more accessible are St. Mary’s Glacier west of Denver, Arapaho Glacier west of Boulder (the Boulder Chamber of Commerce sponsors a festive hike to Arapaho Glacier every August), and Tyndall Glacier in Rocky Mountain National Park. The Ice Age brought drastic changes also to the landscape below 8,000 feet elevation. Heavily loaded with glacial debris, mountain streams disgorged coarse sands and gravels along the mountain front and in the intermontane basins. As the glaciers melted after each period of expansion, the swollen streams cut deeply into their former deposits and into much older rocks as well. Royal Gorge, the Black Canyon of the Gunnison, and many of the deep, colorful canyons of the Plateau Province were cut or at least deepened by these waters. The canyons along the east face of the mountains—Big Thompson, Boulder, Clear Creek, and others—were also deepened and sharpened by the rushing ice-fed torrents. On the prairies, rivers dumping their loads of sand covered the older rocks. Sand dunes developed along the river channels. Bones and huge tusks of hairy mammoths were sometimes buried in these soft deposits; now they are occasionally revealed as the dune and river sands are washed or blown away by continuing erosion. About 20,000 years ago, man arrived in Colorado. Soon after this, the water supply of the valleys diminished greatly, and erosion slowed down correspondingly. The climate gradually became semiarid to arid. Many features of the natural scene were much as they must have been a century ago, without the highways, dams, and television aerials of today. Buffalo and many smaller types of game roamed the plains and foothills; deer, elk, and bighorn sheep were plentiful in the mountains. Nomadic tribes camped and hunted in both mountain and prairie. In the western part of the state, homes could be built in the shelter of great caves, as at Mesa Verde, and game could be supplemented with corn and squash planted on plateau surfaces. Several features of Colorado scenery changed with increasing aridity. The glaciers of course were gone or nearly gone. Streams were no longer the violent torrents they had been. Many mountain lakes, filled with sediment and vegetation, became instead mountain meadows. And the once fertile intermontane valleys became deserts.