Figure 56. The final 3 million years on our yardstick of time, enlarged to show approximate dates of major events.
| THE LAST HUNDREDTHS OF AN INCH OF THE YARDSTICK | ABSOLUTE TIME (Years ago) | IMPORTANT EVENTS |
| 0 | 0 | Last glaciation followed by faulting |
| ¹/₁₀₀₀ | 50000 | Second glaciation |
| ²/₁₀₀₀ | 100000 | ?—First glaciation |
| ⁶/₁₀₀₀ | 700000 | ?—Second Quaternary lake |
| ⁸/₁₀₀₀ | 1 million | ?—Tilting and faulting of southern part of Jackson Hole |
| ¹¹/₁₀₀₀ | 1.3 million | ?—First Quaternary lake |
| ¹²/₁₀₀₀ | 1.5 million | } Complex series of volcanic eruptions in southern Jackson Hole |
| ¹⁵/₁₀₀₀ | 1.9 million | } |
| ¹⁶/₁₀₀₀ | 2 million | ?—Development of Hoback normal fault |
| ²/₁₀₀ | 2.5 million | Eruption of welded tuff in Bivouac Formation |
| ²⁴/₁₀₀₀ | 3 million |
QUATERNARY—TIME OF ICE, MORE LAKES, AND CONTINUED CRUSTAL DISTURBANCE
The Quaternary Period is represented by less than 15-thousandths of the last inch on our yardstick of time ([fig. 56]) and the entire Ice Age takes up less than 2-thousandths of an inch (less than the thickness of this page). Nevertheless, the spectacular effects of various forces of nature on the Teton landscape during this short interval of time are of such significance that they warrant a separate discussion. The role of glaciers in carving the rugged Teton peaks and shaping the adjacent valleys was mentioned in the first part of this booklet, but is discussed in more detail here. The magnitude and complexity of crustal movements increased during the final 2 million years of time—so much so that the beginning of Quaternary time has not yet been identified with any single event. [Figure 56] shows the major events described below.
Hoback normal fault
The Hoback normal fault, 30 miles long, with a mile or more displacement, developed in the southernmost part of Jackson Hole about 2 million years ago. This fault is on the east side of the valley. Thus, the valley block was downdropped between this fault and the Teton fault that borders the west side.
Volcanic activity
During or shortly after major movement on the Hoback fault, and perhaps related to it, there was a complex series of volcanic eruptions west and north of the town of Jackson, along the south boundary of the park. In rapid succession, lavas of many types, with a combined thickness of more than 1,000 feet, were extruded and volcanic plugs intruded into the near-surface sedimentary rocks. These volcanic rocks can be seen on the East and West Gros Ventre Buttes.
There are no active volcanoes in the Teton region today and no postglacial lava flows or cinder cones. Five miles north of Grand Teton National Park are boiling springs (Flagg Ranch hot springs) that are associated with the youngest (late Quaternary) lavas in southern Yellowstone Park. Elsewhere in Jackson Hole are a number of lukewarm springs but their relation to volcanic rocks has not been determined.