Paleontology and geology
The Permian began with broad river floodplains lying north of a coastline that ran east-west across part of southern New Mexico (at about the latitude of Las Cruces). Drier climates followed, and dune fields appeared, but were eventually replaced by shallow, salty seas. More normal marine conditions returned during the middle of the Permian and a huge reef began to develop at El Capitan in southeastern New Mexico. Brachiopods, clams, and ammonites are typical marine fossils from this time period. This map indicates an extensive exposure of Permian rocks in the south-central portion of the state with smaller exposures to the west.