One of my blog posts from a number of weeks ago prompted this inquiry:"About 15 miles south of Holbrook, AZ there are extremely deep fissures (referred to locally as "the Cracks") They range from just a few inches wide to several feet wide, but some seem to go down for a hundred feet. When you drop a rock in the crack you can hear it bouncing down the hole for an extremely long distance. Are these formed from seizmic [sic] activity?"
Actually, this area is pretty quiet seismically. But it is underlain by extensive salt and other evaporate minerals. As these deposits were dissolved over time, the overlying rocks dropped down, creating a broad anticlinal fold. This resulted in extension cracks over the top of the folded layers which have eroded into The Sinks and other similar features in the region.
We have a publication that describes the fractures and related topics with striking photos including the one above.
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