More than 700 million years elapsed between intrusion of the black dikes and deposition of the first Paleozic sedimentary rocks—a longer period of time than has elapsed since the beginning of the Paleozic Era. During this enormous interval the Precambrian rocks were uplifted, exposed to erosion, and gradually worn to a nearly featureless plain, perhaps somewhat resembling the vast flat areas in which similar Precambrian rocks are now exposed in central and eastern Canada. At the close of Precambrian time, about 600 million years ago, the plain slowly floundered and the site of the future Teton Range disappeared beneath shallow seas that were to wash across it intermittently for the next 500 million years. It is to the sediments deposited in these seas that we turn to read the next chapter in the geologic story of the Teton Range. [Illustration: Figure 30. _A glance at the yardstick. The geologic time scale shows positions of principal events recorded in the Precambrian rocks of the Tetons._] ABSOLUTE TIME (Years ago) INCHES Beginning of the Paleozoic. First abundant fossils → 4 1 billion 8 Maximum age of black dikes → 10 Oldest known fossils 15 2 billion 16 Old granite and pegmatite 20 3 billion 24 Gneisses and schists formed sometime in this interval 20-27 Oldest dated rocks → 28 4 billion → 32 Minimum age of the earth 36 THE PALEOZOIC ERA—TIME OF LONG-VANISHED SEAS AND THE DEVELOPMENT OF LIFE The Paleozoic sequence North, west, and south of the highest Teton peaks the soaring spires and knife-edge ridges of Precambrian rock give way to rounded spurs and lower flat-topped summits, whose slopes are palisaded by continuous gray cliffs that resemble the battlements of some ancient and long-abandoned fortress (fig. 31). As mentioned previously, the cliffs are the projecting edges of layers of sedimentary rocks of Paleozoic age that accumulated in or along the margins of shallow seas. At one time the layers formed a thick unbroken, nearly horizontal blanket across the Precambrian basement rocks, but subsequent uplift of the eastern edge of the Teton fault block tilted them westward. They were then stripped from the highest peaks.