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The Cambrian Tonto Group is the three-member sequence of geologic formations that represent the basal section of Paleozoic rocks in the Grand Canyon. The group is about thick.〔Chronic, H (2001) ''Roadside Geology of Arizona.'' The Mountaineers Books, Seattle, Washington. (softcover, ISBN 978-0-87842-147-3) p. 179.〕 The base unit, the Cambrian Tapeats Sandstone was deposited upon the erosion surface of the Vishnu Basement Rocks, which is found in Granite Gorge (the Inner Gorge). The erosion resistant Tapeats Sandstone forms the platform, called Tonto Platform, that the two less erosion resistant upper layers, the Bright Angel Shale and Muav Limestone, rest on. The Tonto Trail is a mostly horizontal trail on the south side of Granite Gorge. The horizontal Tonto Group units are laid upon the Vishnu Basement Rocks above an angular unconformity as the Vishnu Basement Rocks have a dip of about 15 degrees. This erosion unconformity prior to the deposition of the Tapeats upon the tilted Vishnu Basement Rocks is about 1,000 million years (1.0 billion years), and is called the Great Unconformity. ==Geologic sequence== The units of the Tonto Group: (~544-505 Mya) * 3--Muav Limestone * 2--Bright Angel Shale * 1--Tapeats Sandstone File:Tonto-trail.jpg|View of Tower of Set peak-(and sub-unit cliff section, attached at SE), from Tonto Trail, Granite Gorge, north of Mohave Point, Grand Canyon Village, South Rim. The peak is behind and separated from a cliff unit (with small prominence), in front-(photo center, right, Tower of Set (peak) to its left). Vertical erosion in cliff of Redwall Limestone, upon horizontal-layered Muav Limestne cliff, (bottom 25 percent of cliff).〔Lucchitta, I. (2001) ''Hiking Arizona's Geology,'' Mountaineers's Books, Seattle, Washington. (softcover, ISBN 0-89886-730-4) pp. 62-68, Kaibab Trail, pp. 66-67, photo and Fig. 10, ''View to the northwest from the top of the Redwall Limestone along the (S.) Kaibab Trail. The sketch identifies the geologic units visible from here.''〕 The Tapeats Sandstone sits in foreground on Granite Gorge, (75 ft thick?), and is seen as thinly-bedded, in a multi-layered sequence of small beds. The slope-former above is the Bright Angel Shale, and is easily seen with thin, inter-bedding, as well as one resistant cliff unit. The Redwall Limestone cliff section in Grand Canyon is about thick.〔 (high resolution photo) 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「Tonto Group」の詳細全文を読む スポンサード リンク
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