The close of the Precambrian—end of the beginning
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.
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 |