Groves of aspen colonize the eastern prairies in areas where there is sufficient water and protection from wind. These aspen parklands are important havens for animals. Wherever two differing communities interact, a phenomenon known as “edge effect” occurs. Here wildlife exists in abundance; the animals that favor forest cover mingle freely with those that prefer open areas. Aspen groves—supporting grasses, herbs, and shrubs beneath their thin canopies—are favored haunts for grouse, varying hare, deer, and wapiti, all of which find among the trees abundant food, shelter and concealment. Populations of insects, small mammals, and birds, which are high for the same reasons, attract a wide range of predators. Isolated aspen groves are characteristically dome-shaped. Because aspens are capable of reproducing themselves vegetatively, the grove slowly expands outward from the parent tree. As a result, most of these groves are either exclusively male or exclusively female. Since quick-growing aspens provide a bountiful food source for beaver, streams near these trees are often dammed by the rodents flooding lowlands and creating additional habitat in the form of willow flats. Another “edge effect” is established, attracting animals found near water. Waterfowl, marsh birds, moose, mink, muskrat, skunks, amphibians, and many others find such areas to their liking. ■ Before the appearance of the white man, these eastern prairies were a paradise for animals. Once, on the summit of Rising Wolf, light-headed from the climb and the view of endless prairie, I fancied that I saw that vast, undisturbed animal panorama spread before me. Principally there were the bison, darkening the uneven land. Pronghorn bands flashed white on ridgetops, and moose moved through the long fingers of willow that extended eastward with the rivers. Caribou and wolves inhabited the shadows. Among vast cities of prairie dogs, swift fox and grizzly roamed. There were the clamorings of sandhill crane, and white clouds of trumpeter swans. This land, endowed with a wealth of wild grass, wore its wilderness well. The Forest On Gunsight Pass, the rain lancing down, I found a sharpedged rock that split the continent in two. On both sides the rain rivulets ran down, a fraction of an inch determining the stream’s destination: Pacific or Atlantic. The Continental Divide is a mighty barrier, a line of consequence that does more than determine watersheds. Its effect in Glacier is dramatic, as a look at the forests will reveal. Obstructing the eastward flow of the moisture-laden Pacific winds, the Divide extracts a heavy annual tribute of precipitation from the air mass, forcing it to rise up the mountain chain, where it cools and condenses. Chief benefactors are the low western valleys, which respond with a lush growth of Pacific coastal-type forests.