The Boulder Creek, Pikes Peak, and Silver Plume Granites cut through the metamorphic rocks, and are therefore younger. They represent pulses of molten rock forced upward from deep within the crust, probably during three separate episodes of mountain building. As each set of mountains was formed, it was worn down, perhaps to low rolling hills, perhaps to flat plains almost at sea level, and partially or entirely covered with thick layers of sediment. Each time, another mountain building episode followed, with new intrusions of granite and new metamorphism of the pre-existing rocks. Each succeeding period of metamorphism and mountain building further changed the nature of the rocks involved, complicating the patterns of folding and faulting, adding recrystallization to recrystallization, until the oldest of the rocks bore little trace of their original sedimentary nature. In general, the rocks that are oldest were most altered by the repeated metamorphism, while the younger rocks were less altered. [Illustration: The Black Canyon of the Gunnison River is one of the state’s deep and spectacular chasms. Canyon walls are of Precambrian gneiss intruded by many dikes and highly fractured by later uplifts. The flat upper surface of the Precambrian rocks represents an ancient plain on which, during Jurassic time, the dinosaur-bearing Morrison Formation was deposited. (John Chronic photo)] The Precambrian Era ended with a long period of erosion, a period known to geologists as the Lipalian Interval. During this time, over almost the entire world there was no mountain building. The land lay sleeping, subject only to the forces of erosion. The last mountains were flattened nearly to sea level. Slow, sluggish streams and rivers carried sand and mud toward the oceans—oceans in which perhaps primitive, soft-bodied organisms, with no hard parts to be preserved as fossils, were beginning to evolve. On the continents, the time of intense metamorphism was over; most rocks of later eras are preserved today in pretty much their original state. The boundary between the Precambrian and later rocks is normally well defined. It is visible at many places in Colorado: in Williams Canyon near Colorado Springs, in Glenwood Canyon, near Red Rocks west of Denver, just west of La Veta Pass, at the top of Royal Gorge and the Black Canyon of the Gunnison. At most of these localities it is a smoothly beveled surface, with highly contorted Precambrian rocks below it and flat-lying Paleozoic sediments above it. Near Red Rocks and La Veta Pass, the same relationship prevails, but the entire contact, and the rocks above and below it, have been steeply tilted by the uplift of the present mountains.