[Illustration: During the last Ice Age, elephant-like mastodons roamed Colorado. As present-day erosion removes sediments, bones, teeth, and tusks are frequently exposed, especially in the Prairie Province. (C. R. Knight painting, courtesy American Museum of Natural History)] [Illustration: Mastodon] On the eastern side of the San Luis Valley, the Great Sand Dunes developed at this time. These dunes nestle against the Sangre de Cristo Range, where strong southwesterly winds blowing across the wide valley tend to funnel toward Mosca and Music Passes. These winds lift loads of sand from the lightly vegetated valley floor, and drop it as they rise over the mountains. Where the sand is dropped, the dunes have formed. They rise to about 700 feet above the valley floor, and cover about forty square miles. The low rainfall of the area, seven to eight inches per year, keeps vegetation from creeping over the dunes and makes them a most distinctive feature of Colorado, a lesson in geology in the making. * * * * * * * * Geologic processes in Colorado now seem to be much reduced from what they were a few thousand years ago. Reduction in rainfall has led to reduced erosion. Mountain-building, having reached a climax in Tertiary time, has declined markedly. However, we find evidence that volcanism has occurred within the last few thousand years and faulting within the last few hundred, and Colorado streams rise after sudden mountain storms to approximate the violent torrents of glacial times. Colorado’s scenery, fashioned during some three billion years of earth history, is ever changing. [Illustration: The Great Sand Dunes of Colorado were formed during Pleistocene and Recent time by deposition of quartz sand lifted from unconsolidated alluvial deposits in the San Luis Valley. The highest of the dunes rises 700 feet above the adjacent valley floor. (John Chronic photo)] III Geology and Man in Colorado Colorado’s first permanent settlers arrived in 1858, when gold was discovered in river sands near what is now the city of Denver. The ensuing gold rush, coming ten years after the rush to California, rivalled it in fury and brought sudden wealth to lucky miners and the adventurous merchants who grubstaked them. Several hundred mining towns or “camps” sprang into existence almost overnight, their sites determined by the geology of the mountain areas. The cities of Denver, Boulder, and Golden were established as milling and shipping centers for the products of the mines. In 1876 the now-wealthy area, previously part of Kansas Territory, became the State of Colorado.