Outcrops are not prominent, owing to the small amount of weather-resisting materials. The sandy clays are commonly of a deep red color mottled with spots of light gray. A white limestone is sometimes present near the middle of these deposits, and gypsum beds are included locally. The formation is often indicated only by red soil in the depressions between ridges. Few fossils have been reported. _Lyons._ This formation is usually prominent as the eastern wall of the uplifted Red Rocks series. In some localities it forms a ridge of pink or white sandstone distinctly separated from the older sediments to the west. Very few fossils are found. _Fountain._ Exposures usually are brown to red in color, though sometimes a dirty white. The prominent rocks are rather coarse sandstone, commonly with a gritty texture due to the angular character of the sand or gravel from which they were made. These are the westernmost of the Red Beds and the oldest of the uplifted sedimentary rocks bordering the foothills in most of our area. Fossils have been found in the formation, but it is practically barren for the territory here considered. * * * * * * * * This geological section also illustrates a method of dating crustal movements and the birth of mountain ranges, for the folding of the strata along the flanks of the Rocky Mountains has a great deal of significance in this connection. The sedimentary layers were originally deposited over much of the present mountain area in a horizontal position, and only those formations in existence at the time could be distorted by the upheavals which produced the new elevations. Of the series generally involved in the movement the Laramie beds are the youngest. Since these beds had not been formed until near the close of the Cretaceous period it is to be assumed that the mountains must be of more recent date, younger than the topmost of the deformed beds and at least as old as the lowermost of the undisturbed formations overlying them. Some disturbance is evident also in the Arapahoe and Denver beds which overlie the Laramie, but this is believed to have occurred sometime after the occasion of the first great uplift. Volcanic materials in these beds lead to the belief that the sediments were deposited during a period of volcanic activity brought on by the crustal folding which terminated the Mesozoic era. Hence the conclusion arises that the age of the Denver and Arapahoe beds must coincide closely with some of the earlier stages in the history of the mountain system. This interval is often referred to as Post-Laramie time. BEFORE THE AGE OF REPTILES THE PRE-CAMBRIAN COMPLEX