Abstract

The investigation and mapping of the Algonkian rock of the Grand Canyon of Arizona disclosed that a group of sediments should be separated from the Unkar and Chuar groups described by C. D. Walcott in 1895. This intermediate group, called Nankoweap, includes the lower member of the Chuar group as described by Walcott and the upper portion of his Unkar group. The sediments of the Nankoweap group are 330 feet thick and are separated from both the Unkar and Chuar groups by erosional unconformities. A fossil jellyfish medusa found in the Nankoweap sediments suggests that a portion of the Nankoweap has been deposited in an ocean, bay, or estuary.

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