During these episodes of mountain building, erosion, and deposition, the dinosaurs became extinct all over the world. The “Age of Mammals” was about to begin.

TERTIARY—TIME OF MAMMALS, MOUNTAINS, LAKES, AND VOLCANOES

Figure 44. The last inch of the yardstick, enlarged to show subdivisions of the Cenozoic Era.

STRATIGRAPHIC SCALE THE LAST INCH OF THE YARDSTICK ABSOLUTE TIME (million years ago)
CENOZOIC
QUATERNARY
Recent and Pleistocene 0 0
TERTIARY
Pliocene 0 0
Miocene ⅛ 12
Oligocene ¼ 25
Eocene ⅜ 40
Paleocene ⁷/₁₆ 55
MESOZOIC
CRETACEOUS ½ 65

The Cenozoic ([table 1]), last and shortest of the geologic eras, comprises the Tertiary and Quaternary Periods. It began about 65 million years ago and is represented by only the final one-half inch of our imaginary yardstick of time ([fig. 19]). Nevertheless, it is the era during which the Tetons rose in their present form and the landscape was sculptured into the panorama of beauty that we now see. In order to show the many Tertiary and Quaternary events in the Teton region, it is necessary to enlarge greatly the last part of the yardstick ([fig. 44]). There are two reasons for the extraordinarily clear and complete record. First, the Teton region was a relatively active part of the earth’s crust, characterized by many downdropped blocks. The number of events is great and their records are preserved in sediments trapped in the subsiding basins. Second, the geologically recent past is much easier to see than the far dimmer, distant past; the rocks that record later events are fresher, less altered, more complete, and more easily interpreted than are those that tell us of older events.

Table 5.—Cenozoic sedimentary rocks and unconsolidated deposits in the Teton region.

Age Formation Thickness (feet) Description Where exposed
QUATERNARY
Recent
Modern stream, landslide, glacial and talus deposits 0-200± Sand, gravel, and silt along present streams; jumbled broken rock in landslides and on talus slopes; debris around existing glaciers. Floor of Jackson Hole and in canyons and on mountainsides throughout the region.
Pleistocene
Glacial deposits and loess 0-200± Gravel, sand, silt, and glacial debris. Floor of Jackson Hole.
Unnamed upper lake sequence 0-500 Shale, brown-gray, sandstone, and conglomerate. Gros Ventre River Valley.
Unnamed lower lake sequence 0-200 Shale, siltstone, and sandstone, gray, green, and red. National Elk Refuge.
? Pleistocene or Pliocene
Bivouac Formation 0-1,000 Conglomerate, with purplish-gray welded tuff in upper part. Signal Mountain and West Gros Ventre Butte.
TERTIARY
Pliocene
Teewinot Formation 0-6,000 Limestone, tuff, and claystone, white, soft. National Elk Refuge, Blacktail Butte, and eastern margin of Antelope Flat.
Camp Davis Formation 0-5,500 Conglomerate, red and gray, with white tuff, diatomite, and red and white claystone. Southernmost tip of Jackson Hole.
Miocene
Colter Formation 0-7,000 Volcanic conglomerate, tuff, and sandstone, white to green-brown, with locally-derived basalt and andesite rock fragments. Pilgrim and Ditch Creeks, and north end of Teton Range.
Oligocene
Wiggins Formation 0-3,000 Volcanic conglomerate, gray to brown, with white tuff layers. Eastern margin of Jackson Hole.
Eocene
Unnamed upper and middle Eocene sequence 0-1,000 Tuff, conglomerate, sandstone, and claystone, green, underlain by variegated claystone and quartzite pebble conglomerate. Eastern margin of Jackson Hole.
Wind River and Indian Meadows Formations 2,000-3,000 Claystone and sandstone, variegated, and locally-derived conglomerate; persistent coal and gray shale zone in middle. Eastern margin of Jackson Hole.
Paleocene
Unnamed greenish-gray and brown sandstone and claystone sequence 1,000-2,000 Sandstone and claystone, greenish-gray and brown, intertonguing at base with quartzite pebble conglomerate. Eastern margin of Jackson Hole.
Pinyon Conglomerate 500-5,000 Conglomerate, brown, chiefly of rounded quartzite; coal and claystone locally at base. Eastern part of Jackson Hole, Mt. Leidy and Pinyon Peak Highlands, and north end of Teton Range.