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The Cambrian Tapeats Sandstone is the lower geologic unit, about thick,〔Chronic, H (1983) ''Roadside Geology of Arizona.'' The Mountaineers Books, Seattle, Washington. (softcover, ISBN 978-0-87842-147-3) p. 179.〕 at its maximum, of the 3-member Tonto Group. The Tapeats Sandstone is the highly erosion-resistant unit laid upon the Vishnu Basement Rocks in the central, parts of east, and parts of west Grand Canyon, Arizona. The unit comprises the 'base horizontal unit' of the platform around Granite Gorge (Inner Gorge) on the Colorado River, and because of its hardness, creates the Tonto Platform upon which the two ''slope-forming'', highly erodable units, the Muav Limestone, and Bright Angel Shale lie. The Tapeats Sandstone and the Tonto Platform follow the Colorado River, and surrounding tributary canyons, creeks, watercourses, or washes, in a dendritic fashion, much like the branches of a tree. The Tapeats unit is the bottom member of the Tonto Group, a typical ''marine transgression series'' of sandstone-(conglomerate)-shale-limestone, all part of a paleo sea, initially adjacent to land, the source of the Tapeats rocks-(conglomerates) and sand; (a regressing sequence has the reverse order). The Tapeats Sea at the early Devonian ceased to deposit more Muav Limestone, and a period of erosion ensued, a ''deposition unconformity''. The Tapeats Sandstone was laid upon the Vishnu Basement Rocks, after an unconformity of erosion, the Great Unconformity, of 1,000 million years (1.0 billion). Besides the ''erosion unconformity'', the Unkar Group of the basement rocks are also at an ''angular unconformity'', being an 8-member sequence tilted at 45 degrees. The horizontal Tonto Platform has hiking trails that cross it from the South Rim to North Rim, Grand Canyon for instance. The extensive Tonto Trail lies on parts of the Tapeats Sandstone, and the platform on the south side of Granite Gorge. ==Geologic sequence== The units of the Tonto Group: * 3--Muav Limestone * 2--Bright Angel Shale * 1--Tapeats Sandstone (start of transgression series) 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「Tapeats Sandstone」の詳細全文を読む スポンサード リンク
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