"the ensuing Carretas Plain that was once a seafloor"
Alto Plano plane between Sonora and Chihuahua
Permission Granted by Copyright Owner for Use on Book DrumAlto Plano plane between Sonora and Chihuahua - Credit: RAEchel Running (www.raechelrunning.com)

McCarthy is likely referring to the Llano de Carretas (Plain of Carts) which extends west from the Rio Casas Grandes at Janos to the high passes to Sonora.