Spring thunderstorms bring rain to Chisos Mountains and the Chihuahuan Desert in Big Bend. This view is looking north from the Mariscal Mine area. Low buttes capped with remnants Tertiary-age volcanic flows are to the right. These lava flows rest on top of Late Cretaceous-age sedimentary rocks (Aguja Formation). The Chisos Mountains are the eroded remnant of massive igneous plutons and associated volcanic rocks that formed in the early middle Tertiary. Igneous intrusions and volcanism occurred throughout the southern Rocky Mountain region extending from New Mexico, southward through Texas into Mexico throughout this same period.