The older of the two, revealed in the rocks exposed in the canyon's walls, provide a record of several eras. There is an extensive but incomplete record of the Paleozoic Era (550-250 million years ago), as well as "scattered remnants of Precambrian rocks as old as 2000 million years Mesozoic and Cenozoic rocks (250 million years old to the present) are largely missing at Grand Canyon (they've either been worn away or were never deposited)" (National Park Service, 2000; p. index).
A University of Akron geologist - one whose primary interest in the canyon is its structure and formation - writes, "Poets and artist have tried to capture its beauty and its size, but the only way to truly appreciate it, is to stand on the rim and see it for yourself" (Foos, 2000; p. grand). There has been much mention of the visible colors and the rugged beauty of the cliffs, slopes and peaks formed from erosion of different rock formations that weather at different rates, but the first feature mentioned by most is the Grand Canyon's immense size. Its average width is about ten miles; it is 277 miles long and averages one mile deep along its entire length.
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Stratigraphy.
As stated, the rocks of the Grand Canyon range from Proterozoic (Precambrian) to Permian, with breaks in the geological record represented in the lack of Ordovician, Silurian, Mesozoic and Cenozoic rocks. These breaks are known as unconformities; a hiatus is the period that "should" be present but is not. Within the Grand Canyon, there are "14 major unconformities in the stratigraphic section exposed in the Grand Canyon" (Foos, 2000; p. grand).
Unconformities are of three types, all of which are represented in the Grand Canyon. An unconformity occurs where "sedimentary rocks are deposited directly on igneous and metamorphic basement" (Foos, 2000; p. grand). A non-conformity results from the interface of the Precambrian Vishnu Schist and the Precambrian Grand Canyon Super group and Cambrian Tapeats Sandstone.
