The Hermosa formation of Pennsylvanian age, exposed in the San Juan Canyon where it transects

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the Monument upwarp in southeastern Utah, contains several biostromal layers, one of which changes laterally into small bioherms with typical reef-limestone breccia making up the greater part of the deposit.

A bioherm excellently exposed in the San Juan Canyon walls was examined just above river level in Sec. 4, T. 42 S., R. 18 E., between Mendenhall Loop and Honaker Train. At least eight bioherms have been breached by the San Juan River, several of which are exposed on only one canyon wall. The reef limestones are present approximately two-thirds of the Hermosa thickness below the Hermosa-Rico gradational contact, and the fossils suggest that the bioherms are of Des Moines age.

Regionally, the biohermal trends may be controlled by a shelf related to the Paradox-Salt Valley Basin in southeastern Utah and southwestern Colorado. Valid predictions as to possible distribution of these reefing limestones must be based on further surface examination, subsurface analysis, and an appraisal of regional controls exerted by the Paradox evaporite basin in relation to the Colorado Mountain highlands (Uncompahgre).

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