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The Chugwater Formation is a mapped bedrock unit consisting primarily of red sandstone, in the states of Wyoming, Montana, and Colorado in the USA. It is recognized as a geologic formation in Colorado and Montana, but as a Group (set of formations) in Wyoming . Due to its lack of fossils and its presence below the highly studied Morrison Formation, the Chugwater receives little attention. ==Description== The most noticeable feature on a large scale is the brick-red color, caused by oxidation of iron minerals in the rock. This color is periodically interrupted by streaks and spots of reduced iron, a light bluish-gray shade. Near the top of the formation is a thick layer of gypsum of very high quality. The whole rock is interrupted by gypsum veins as well as having a disrupted texture because of the precipitation of gypsum crystals after deposition of the rock. The Chugwater consists mainly of siltstone and shales with interspersed sandstones. While this composition will lend itself to breaking easily and not outcropping, the shear size of the formation causes there to be large outcrops. Though most sedimentary structures are not visible due to the gypsum interruption, there are a few examples of ripple marks and occasional crossbedding. Examples of infilled mudcracks of irregular shape, are common. The main constituent of the Chugwater is quartz, which helps to account for its unusual hardness. Feldspars are also present, though in lesser amounts, as are hematite and gypsum. Small flecks of a dark, opaque mineral have been tentatively identified as biotite. The minerals are cemented by a non-carbonate matrix, probably siliceous. The Chugwater is a prime example of red beds, commonly deposited during the Permian and Triassic periods. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chugwater Formation」の詳細全文を読む スポンサード リンク
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