Tonto Group
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. 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 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 upon.
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, and is called the Great Unconformity.Geologic sequence
The units of the Tonto Group:
- 3 Muav Limestone
- 2 Bright Angel Shale
- 1 Tapeats Sandstone
Popular publications
- Arizona Road & Recreation Atlas, Benchmark Maps, 2nd Edition, c. 1998, 112 pages, pp. 28–29, 62-63.
- Arizona DeLorme Atlas & Gazetteer, 5th Edition, c. 2002, 76 pages, pp. 24, 32.
- Chronic, Halka Roadside Geology of Arizona, c. 1983, 23rd printing, Mountain Press Publishing Co. 322 pages. pp. 229–232-, pp. 179–180.
- Lucchitta, I. Hiking Arizona's Geology, Ivo Lucchitta, c 2001, Mountaineers's Books.
