The formation is transitional with the underlying Glorieta Sandstone or Yeso Group. Where the basal sandstone beds are a minor component compared with marine limestone, these beds are assigned to the Rio Bonito Member of the San Andres Formation and no Glorieta Sandstone is mapped. Where the basal sandstones contain little or no marine limestone, the lower beds are mapped as Glorieta Sandstone and no Rio Bonita Member is mapped. This transition from clastic to marine beds represents a late Leonardian marine transgression that was the last to reach northern New Mexico for the next 150 million years. The remaining members are the Bonney Canyon Member, a relatively thin section of thin-bedded limestone with local gypsum beds, and the upper Fourmile Draw Member, which is primarily marine carbonates. The Fourmile Draw grades into the Brushy Canyon and Cherry Canyon Formations in the Delaware Basin and contains considerable gypsum in the Guadalupe Mountains. The entire San Andres Formation tends to become more dolomitic to the north.