The Chisos Basin has been hollowed out of volcanic rock by stream erosion. The peaks that ring the Basin all came into being when molten rock squeezed up under enormous pressure from deep within the Earth. Some of the red hot stuff poured out over the land surface in lava flows which cooled so quickly that they cracked in long vertical fissures. Then as the ages passed, joints toppled and square-faced peaks, buttresses, and free-standing spires emerged. So the Basin wall took shape from Casa Grande southward through Emory Peak. Later on, more molten rock pushed up from below, bulging the surface rocks upward without breaking through. Again the eons passed and the softer surface rocks wore away, exposing the dome-shaped peaks which now rim the Basin to the north and west. Today, loose rocks and clays still inch downhill toward the Window. All Basin runoff heads for this chute, and when it storms in the heights, the dry waterfall turns into a torrent, with boulders bouncing along like so many corks. Growling and grumbling, the big rocks plunge over the pouroff in a 67-meter (220-foot) free fall, coming to Earth in a great rubble pile below. Pinyon pines grow abundantly across both Mexico and the American Southwest. In the Chisos you’ll find them almost anywhere above 1,500 meters (4,800 feet) of elevation, and at lower elevations they will be the only pines. Short of trunk, with spreading lower branches, egg-sized cones, and short, slender, bluish-green needles, pinyons range from dwarf size to a tree 15 meters (50 feet) tall. Many birds and mammals eat its delicious nuts. As elevation increases you may find fewer junipers, while more pinyons appear on the open slopes and more oaks along drainages. The deciduous Graves oak requires more moisture than other oaks, so you find it putting forth its shiny, dark green leaves in high moist canyons. The Emory oak also prefers high drainages but it grows at slightly lower elevations. It has small lance-shaped leaves. The Chisos oak, a small, graceful tree with narrow, leathery leaves, requires a high water table. In all the world it grows only in the Chisos Mountains’ Blue Creek Canyon. [Illustration: Mule deer frequent the park’s lower, drier elevations.] [Illustration: The Sierra del Carmen whitetail haunts the Chisos Mountains. It has much smaller ears than the mule deer and bears the characteristic flag tail. Isolation allowed this sub-species of whitetail to develop. These deer live only in the Chisos and across the river in the Sierra del Carmen.]