Every large high-altitude canyon in what is now Rocky Mountain National Park became filled with snow, much of which, under pressure, turned to ice. The glaciers thus formed, moving under their own great weight—slowly, but with tremendous power—broadened, deepened, and straightened the twists and turns of the original river-cut valleys, and, bit by bit, scooped out bowls, or cirques, at the glacier sources. These glaciers quarried and removed untold millions of tons of rocks from the upper reaches. Many of the cliffs and lakes of the park are the results of excavating done by the mountain glaciers. These glaciers were entirely local; they did not extend down to the plains in this region. At what is now an altitude of about 8,200 feet—just below present-day Moraine Park, Horseshoe Park, Wild Basin, and a few miles below Grand Lake in the Colorado River valley—the glacier fronts melted as fast as the ice advanced. It was there that most of the rock debris plucked from higher up was dropped. Piles of rock are scattered over most of the meadowlands of this general altitude. These rock deposits are called moraines—ridges and heaps, or scattered masses, of unsorted rock debris dumped where they settled from the melting ice. Classic examples of moraines may be seen in Moraine Park, named for these special features. You can learn more about them at the Moraine Park Visitor Center. When climatic changes caused the glaciers to melt back faster than they advanced, the moraines, like modern dams, formed lakes behind them whenever stream drainage from the shrinking glaciers was checked. Several such lakes, now silted in and changed to green meadows, occupied lower regions in the park. There are good examples of these lakes in Horseshoe Park and Moraine Park. [Illustration: Bierstadt Ridge, a huge glacial moraine, seen en route to Bear Lake.] The glaciers invaded the park valleys several times during the ice age. Usually, two distinguishable ages of moraines can be seen; the older and more extensive one is made up of well-weathered, “rotten” boulders and finer material, while the newer ones are scarcely altered at all. It is thought that the last great glaciers retreated only some 12,000 years ago. Indians lived on the plains at that time! [Illustration: Andrews Glacier, late in season. Note crevasse in upper middle of glacier.] It is by no means certain that the glacier age is entirely a thing of the past. Five small glaciers, or “glacierets,” of geologically recent origin—Taylor, Andrews, Tyndall, Rowe, and Sprague—exist today in the park. They are much smaller than the earlier glaciers; but they are ice masses, and they are moving (though very slowly); so they are glaciers by definition. They are accessible only by arduous foot travel, but the first three are visible from heavily traveled roads and trails of the park.