The great ice ages that began about one million years ago were marked by the advance and recession of huge ice sheets over vast areas of North America. These continental glaciers, originating in Canada, moved southward, scraping up mantle rock and soil that was dropped in central and southern Minnesota to produce plains and irregular belts of hills. Most of Minnesota's 10,000 lakes lie in such deposits and trace their origin directly or indirectly to glaciation. In the rugged surface extending from Detroit Lakes to Alexandria, where glacial action was particularly vigorous, the lakes are irregular in outline. Elsewhere they may be round, long, wide, narrow, big, little, sun-warmed or ice-cold, shallow and sandy or rocky and deep, mucky and weed-fringed or clear as crystal, with or without islands, inlets, bays, sand bars, beaches or cliffs. Taken together they give Minnesota a water area greater than that of any other state. Many exhibit landscapes of unusual beauty, but all, regardless of location or character, add to Minnesota's most valuable mineral resource - water.