The paper embodies results of a study of surface exposures of Rocky Arroyo, 12 miles northwest of Carlsbad, New Mexico. In the walls of this and adjacent canyons is revealed an abrupt lateral change in lithology in the Seven Rivers formation. A 275-foot section of gypsum with thin beds of dolomitic limestone merges into a thinner section of uniform dolomitic limestone with some sandstone beds. Evidence is presented to show that this change is the result of interfingering of gypsum and dolomitic limestone. Thin beds of the latter extend for some distance into the gypsum. However, thick layers of gypsum between beds of dolomitic limestone end abruptly and their places are taken by zones of red porous loosely crystalline calcitic limestone. These limestone beds become thinne away from the gypsum beds and finally pinch out, so that in the final analysis the gypsum beds are equivalent to bedding planes in the section of dolomitic limestone. Channeling of the canyon walls by present-day streams has produced a striking type of breccia, in which angular blocks of dense light-colored dolomitic limestones are firmly embedded in red crystalline highly calcitic limestone.

The following conclusions are suggested. The lateral change of section does not represent overlap, as has previously been suggested, but is an abrupt lateral gradation. This gradation is not a local phenomenon but occurs in the Seven Rivers formation for an undetermined distance at the same relative position back of the Capitan Reef, the controlling factor of Permian sedimentation in this region. Advance and retreat of the Seven Rivers seas is suggested, with the anhydrite-depositing environment approaching closest to the Capitan Reef in earliest Seven Rivers time and thereafter making shorter advances. The presence of the lateral gradation in the subsurface should be taken into account when correlating well logs penetrating the Seven Rivers section farther to the east.

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