The highest points in New Mexico are in the north-central region. Here, along the backbone of the Sangre de Cristo Mountains, tower Wheeler Peak (13,160 feet above sea level), northeast of Taos, and South Truchas Peak (13,102 feet), northeast of Santa Fe. Snow lingers on these lofty spires all year around except during an especially hot August. Highest peaks—they must be capped by the youngest rocks. But no, the quartzites and gneisses, hard rocks made up of quartz and feldspar, that hold up these pinnacles against the attack of water and ice are among the oldest rocks known in the Southwest—perhaps as much as two billion years old. These Precambrian rocks lay deep beneath the earth’s surface from early Paleozoic time (500 m.y. ago?) until early Neogene time, then were uplifted along great breaks in the earth’s crust, uplifted slowly and intermittently during the span of time from about 20 m.y. ago to perhaps 1 m.y. ago. [Illustration: Figure 4. Gypsum dunes of White Sands] How did these ancient rocks form? What did New Mexico look like during the dawn of geologic history? The record in stone is fragmentary. But about two billion or so years back, thick masses of quartz sandstone were laid down in north-central New Mexico, as well as vast lenses of mud, and some beds of feldspar-rich sandstone. Volcanic activity was intense; huge flows of rhyolite and andesite were poured out over most of the state, and these in turn, along with the muddy and sandy sediments, were intruded by enormous masses of hot granite. Mile-high mountains were formed, and during the stretch of this early geologic time, called the Precambrian Era, the high peaks were eroded by rain, wind, and sun until some were worn down to featureless plains sloping toward the ancient ancestral oceans. [Illustration: Figure 5. Staurolite twin crystals in mica schist]