Dolomitization and anhydritization have occurred in the Mississippian Souris Valley Beds of the Broadview area immediately below the Mississippian-Jurassic erosion surface. These secondary changes are expressed in a relatively impermeable zone of dolomite with a high proportion of anhydrite and some gypsum that ranges in thickness from 8 feet in the south to 123 feet in the north. Evidence for the replacement of dolomite by anhydrite is manifold: inclusions of dolomite are contained within anhydritic bodies; gypsum-anhydrite veins cut the dolomite with irregular contacts; and rhombs of dolomite are corroded by anhydrite. It is contended that, during Watrous Anhydrite times, ground waters rich first in magnesium and then in calcium sulfate percolated through the uncompacted silts of the Watrous Red Beds and attacked the carbonates of the Souris Valley Beds. Accompanying maps and cross sections illustrate the relationship which exists between the Watrous Red Beds and the anyhydritized zone of the Mississippian immediately below the unconformity.

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