Packrat midden data can be used to calculate rates of cliff retreat by relating midden age to the distance between cliff face and midden. Regression analysis using 14 radiocarbon-dated packrat deposits from the Mississippian Redwall Limestone in the eastern Grand Canyon suggests that the Redwall has been retreating at an average rate of 0.45 m/10314C yr. This rate of cliff retreat, which is comparable to other cliff-retreat rates reported from arid environments, implies that the Colorado River cut through the Redwall Limestone in the vicinity of Horseshoe Mesa about 3.7 m.y. B.P.

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