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. [Illustration: Table 5.—Cenozoic sedimentary rocks and unconsolidated deposits in the Teton region.] Age Formation Thickness Description Where exposed (feet) QUATERNARY Recent Modern 0-200± Sand, gravel, and Floor of Jackson stream, silt along present Hole and in canyons landslide, streams; jumbled and on glacial and broken rock in mountainsides talus deposits landslides and on throughout the talus slopes; region. debris around existing glaciers. Pleistocene Glacial 0-200± Gravel, sand, silt, Floor of Jackson deposits and and glacial debris. Hole. loess Unnamed upper 0-500 Shale, brown-gray, Gros Ventre River lake sequence sandstone, and Valley. conglomerate. Unnamed lower 0-200 Shale, siltstone, National Elk Refuge. lake sequence and sandstone, gray, green, and red. ? Pleistocene or Pliocene Bivouac 0-1,000 Conglomerate, with Signal Mountain and Formation purplish-gray West Gros Ventre welded tuff in Butte. upper part. TERTIARY Pliocene Teewinot 0-6,000 Limestone, tuff, National Elk Formation and claystone, Refuge, Blacktail white, soft. Butte, and eastern