[3] The work of Darton and Reeside in 1926[4] established the accepted framework for the stratigraphy of the area, and identified the Capitan Formation as late Permian in age.

Richardson interpreted the Guadalupe Mountains as an east-dipping monocline with a fault on the steep western boundary, and believed El Capitan itself was a product of erosion.

This culminated in the publication by E. Russell Lloyd in 1929 of his interpretation of the Capitan Limestone and associated formations as a gigantic fossil coral reef.

[7] Two months later, a “Symposium on Pennsylvanian and Permian stratigraphy of southwestern United States” appeared in the August, 1929 issue of the Bulletin of the American Association of Petroleum Geologists,[8] which provided a flood of new details on the Capitan reef.

[12] The Capitan Formation consists of compact, massive, light grey to white limestone with minor dolomite.

[11] Richardson (1904) found that the upper and lower beds of the formation were relatively unfossiliferous, but the middle section contained an abundant fossil assemblage unlike any other known at that time.