Southeastern New Mexico appears to have been relatively level with only local hills and vast regions of featureless, stagnant but high plains where erosion slowly ate downward, deposition was slight, and most of the detritus was carried eastward far beyond the state’s borders. The redbeds of the Baca Formation were laid down on the north flank of low mountains that extended intermittently from somewhere near Quemado toward Socorro. Some ancient hills near present-day Sierra Blanca shed rock fragments that accumulated near Capitan as the varicolored Cub Mountain Formation. Deeply eroded uplands northwest of Elephant Butte Reservoir supplied gravels and sands that mingled with andesitic volcanic debris as the upper part of the McRae Formation in central Sierra County. Many of the weathered greenish and purplish volcanic rocks in southwestern New Mexico were extruded at this time, and beneath the surface these molten magmas (hot liquefied rocks) cut into older rocks. Vapors and hot solutions from the magmas are believed to have emplaced some of New Mexico’s vast ore deposits during this time. The last phase of the Paleogene Period, about 25 to 40 m.y. ago, was an earth-shaking time in New Mexico—and the first explosion of an atomic bomb in 1945 on the Jornada del Muerto between Socorro and Carrizozo was a relatively low-energy-yield event compared with the late Paleogene earth movements. Almost the entire southwestern quarter of the state literally exploded, with volcanic eruptions on a grand scale. These lava flows, rock breccias, ashes, pumice, and associated intrusives (molten rocks that did not make it to the surface) form the Datil-Mogollon plateau—at least 100 miles in diameter—as part of the Datil Formation, which locally is miles thick, and made up the main mass of many other ranges near the Mexican border. Sierra Blanca (12,003 feet altitude) northeast of Alamogordo is a huge, isolated volcanic mass of late Paleogene age. [Illustration: Figure 3. East-west cross section of Rio Grande graben near Santa Fe] This widespread volcanic activity continued into the Neogene Period which began about 25 m.y. ago. Rhyolites, pumice, and perlite in the southwest, as well as in other parts of the state, covered wide areas. Mount Taylor, towering up to 11,389 feet near Grants and visible on the western skyline from Albuquerque, is a Neogene volcanic pile, as are parts of the Sangre de Cristo range northeast of Taos. Shiprock and Cabezon Peak, landmarks in northwestern New Mexico, are volcanic necks—the eroded cores of ancient volcanoes.