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. . 4 CJ Anticlines) where the bedrock is hard to erode, rather than flowing around 00 ~~ the two mountains through areas where the sediments 'are less resistant? .... c, It seems to have made extra work for itself. -- Both canyons are the re- sult of a process called "superposition." The anticlines through which the canyons are now cut were formed approximately 60 million years ago at the same time when most of the other structures of the Rocky Mountains had their ~ origin. Following this time of structural deformation (called the laramide Orogeny), most of northwestern Colorado, including the two anticlines, was buried by thousands of feet of younger (Tertiary Period), flat-lying sedi- mentary strata. Then, about five million years ago the Yampa River began to flow and to establish its course on top of the flat Tertiary strata. Following later regional uplift and/or climate change, the river began to cut downward, eroding away the soft Tertiary sediments that covered the anticlines, and eventually reaching the tops of the structures. But by then it was trapped in its own valley, so could not change its direction of flow so as to avoid the hard rock of the anticlines. Thus, the river was forced to cut down through the hard limestone and sandstone layers of Paleozoic and Precambrian age, eventually forming the two canyons. Today ancient Precambrian rocks are exposed in the center of Cross Mountain Canyon, while in the center of Juniper Canyon the river has cut down only as far as strata of mid-Paleozoic age. In sharp contrast to the rugged topography of the two canyons, are the broad valleys immediately upstream from the canyons. In these areas non- resistant strata are present, and the river has been able to erode later- ally, forming broad floodplains over which the river meanders in huge sweeping curves. There is much evidence to show that these meanders are actively shifting back and forth across the floodplain. -. "r-"'