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.
The Paleozoic and younger sedimentary rocks in the Teton region are subdivided into formations, each of which is named. A formation is composed of rock layers which, because of their similar physical characteristics, can be distinguished from overlying and underlying layers. They must be thick enough to be shown on a geologic map. [Table 2] lists the various Paleozoic formations present in and adjacent to Grand Teton National Park and gives their thicknesses and characteristics. These sedimentary rocks are of special interest, for they not only record an important chapter of geologic history but elsewhere in the region they contain petroleum and other mineral deposits.
The Paleozoic rocks can be viewed close at hand from the top of the Teton Village tram ([fig. 32]) on the south boundary of the park. A less accessible but equally spectacular exposure of Paleozoic rocks is in Alaska Basin, along the west margin of the park, where they are stacked like even layers in a gigantic cake ([fig. 33]).
Alaska Basin—site of an outstanding rock and fossil record
Strata in Alaska Basin record with unusual clarity the opening chapters in the chronicle of seas that flowed and ebbed across the future site of the Teton Range during most of the Paleozoic Era. In the various rock layers are inscribed stories of the slow advance and retreat of ancient shorelines, of the storm waves breaking on long-vanished beaches, and of the slow and intricate evolution of the myriads of sea creatures that inhabited these restless waters.
