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~ Geologic Sketch f II I I I The rocks and soils of Colorado's plains, mountains, and plateaus provide a record of geologic history at least a billion and a half years long. As with human history, this record is poor and scanty in its early part but becomes clearer and more detailed as the present time is approached. Ancient rocks comprise a large part of the mountain ranges in Colorado. The two main kinds of these rocks are granite and gneiss, a banded rock with a granitelike grain and dura- bility. These rocks, once some miles within the Earth's crust, can be seen now because of the erosion of vast quantities of covering rock. Gneiss is the older of the two rocks having been invaded by the granite. The granite is not all of the same age, but modern methods of radio- active dating indicate that most of it is 1 to IJ~ billion years old. A long period of destructive erosion followed the period of granite formation, and in some places, such as in the northwestern and southwestern corners of the State, waste products of this process are preserved as thick bodies of sandstone, shale, quartzite, and slate. Approximately a half billion years ago, about the time of the earliest fossils, a great change occurred in the pattern of geologic events. . At that time, Colorado was part of a vast flatland that lay close to sea level. The sea spread over this flatland repeatedly through an interval at least 250 million years long, each time leaving a new layer of fossil-bearing lime- stone or sandstone as. a record of its transgres- sion, This pattern of geologic calm ended with the rise of mountains in an extensive area that corresponded only in part with the mountains pf today. Great quantities of sand, gravel, and mud were washed from these mountains and deposited along the coasts of bordering seas, forming thick layers of sandstone, conglomerate, I' II 48 and shale-many of them reddish in color- which are now exposed in or along the mountain ranges. A little later, similar deposits accumu- lated on land, along with vast blankets of dune sand. About 125 million years ago, the sea returned for the last time and left a layer of mud and sand thousands of feet thick over the whole State. This material, now shale and sandstone, underlies the plains portion and large areas in the western part of Colorado. Great swamps existed along the borders of the retreating sea, arid vegetable matter that accumulated in these ultimately became coal. Most of the mountains of the present Colorado landscape came into existence 60 to 75 million years ago, when, in response to forces within the earth, they bulged upward, breaking and distorting the flat-lying deposits of ocean and stream that had once occupied their site. In many places, molten rock rose toward the surface at the same time, and emanations from the molten masses gave rise to most of the great ore deposits for which Colorado is famed. Later, thousands of square miles in the mountains were buried beneath lavas from many great volcanoes, and the Plains were buried beneath a blanket of alternating volcanic ash and sand and gravel washed from the mountains. Still later, the mountains were re-elevated, and in the cold of the Ice Age, many of them were cloaked by glaciers. The Colorado landscape seen today is a prod- uct of long-continued change. The glaciers that sculptured the mountains have only barely vanished, and the streams that cut away valleys and canyons are still carving. During the prolonged construction and modi- fication of the region now included within the State, many unusual geological features were formed. Among the resulting marvels are the huge caves and passages which honeycomb Colorado National Monument in Mesa County. Erosion produced the fantastic volcanic forma- tions of Wheeler National Monument in the south-central part of the State where cones, massive plugs, and red pinnacles tower above majestic gorges. The great shifting sand dunes in the San Luis Valley are the remnants of a prehistoric desert. -'I :'j ...l....'~ ::~, . .1