Studies on the origin of Montezuma Well and Cave, Arizona

Lange, Arthur

The origins of Montezuma Well are investigated, including the age of porous lime, the material in which the well occurs. Deep-lying salt lens, expanded due to hydration, led to fracturing and the circulation of groundwater. The collapse of the overlying limestone beds, resulted in the formation of the depression of Montezuma Well. Montezuma Well is a natural depression, 140 meters in diameter and 35 meters deep, containing a lake drained by a cave. It is part of Montezuma Castle National Monument, located in the Verde Valley of central Arizona. Studies were conducted by the Western Speleological Institute in conjunction with the Museum of Northern Arizona in order to determine the origin of the Well and Cave, and to add to our understanding of the age of the porous limestone of the Verde formation in which these features occur. Previous theories on the origin of the Well are discussed, and the explanation which regards the Well as a collapse feature resulting from ground-water solution is considered tenable but incomplete. That theory did not explain why other similar depressions do not occur in the vicinity, as is common in karst areas; nor why solution was so favored at this locality. The author proposes the hypothesis that a deep-lying salt lens, similar to others known in the Verde formation, was attacked by ground-water solution, following consolidation of the lake beds. The salt lens, expanding under hydration, encouraged fracturing, and in turn, the circulating of ground-water. Increased solution followed by progressive collapse of overlying limestone beds resulted in the depression. The underwater spring presently feeding the Well is known from recent diving operations. The cave outlet has undergone at least one stage of blocking. The adjacent surface stream, Beaver Creek, eventually tapped a lower-lying ground-water conduit and drained the upper level of the Cave and Well. In time, the narrow isthmus of limestone, through which the Cave stream flows, will disappear by progressive collapse, and the Well will be transformed into a normal, spring-fed valley. Bird and rodent bones were discovered in a primary fissure-fill in the Cave wall. It is hoped that these remains will aid in dating the Verde formation. A detailed description and map of the Well and Cave are included. (Copied verbatim from the report.)