Geological Time Line
Shaping the Yellowstone region
Once visitors were looking at dinosaur footprints in Arizona. “Them footprints,” said their guide, “is 400 million and four years old.”
“But how can you be so precise as all that?”
“Well, (apologetically) of course I don’t really know nothing about it myself. But when I came they told me they was 400 million years old. And I been here four years” (from Joseph Wood Krutch, Voice of the Desert, 1955)
Well, geologists and anthropologists can’t yet be quite that precise. Most of the dates in this geological time line come from analyzing geological samples, measuring radioactive isotopes and their decay products. In the time line, bya means “billion years ago” and mya means “million years ago,” give or take 10 percent.
3.5 to 2.7 bya: Formation of the Precambrian gneiss of the Beartooth Range, seen in Yellowstone’s Lamar Canyon.
360 to 320 mya: Limestones are formed from the shells of sea-dwelling animals, as seen at Pebble Creek in northeastern Yellowstone.
200 mya: The motion of earth’s tectonic plates is bringing the continents into today’s positions. First dinosaurs appear; small mammals live among them.
150 to 70 mya: Shale and sandstone from silts laid down in shallow seas will make up Mt. Everts at Mammoth Hot Springs. Sierra Nevada forms in California.
68 mya: Dinosaurs become extinct, probably as a result of a gigantic meteorite impact in Yucatan, Mexico.
53 to 44 mya: Absaroka Range volcanoes form, and their eruptions cover the area with lava, mudslides, and ash. In the subtropical climate, forest growth and lava flows alternate many times, creating the petrified trees of Specimen Ridge and the Gallatin Range.
7.5 mya: Tetons begin to rise and Jackson Hole begins to sink along a fault; displacement continues today.
7 to 4 mya: First hominid ancestors of humans lived in East Africa.
2.1 mya: First caldera eruption occurs; Huckleberry Ridge tuff accumulates, seen at Golden Gate near Mammoth Hot Springs. Meanwhile, glaciers cover parts of Europe and North America. Ice covers the land and recedes in cycles of 50,000 to 100,000 years’ duration.
1.8 to 1 mya: Man-like apes (Australopithecus africanus) live in Africa.
1.3 mya: Second caldera eruption occurs; Mesa Falls tuff accumulates west of Yellowstone’s Cascade Corner.
800,000 to 200,000 ya: Direct ancestors of today’s humans (Homo erectus) use fire and perfect tools but then are transcended by Homo sapiens.
639,000 ya: Third caldera eruption occurs; Lava Creek tuff accumulates, as at Tuff Cliff between Madison Junction and Gibbon Falls.
160,000 to 130,000 ya: Glaciers cover Yellowstone (the Bull Lake Glaciation, evident in the Madison Valley near West Yellowstone).
Up to 70,000 ya: Lava continues to flow from the magma chamber, filling the caldera and spilling over its rim. Most is rhyolite lava, as at Firehole Falls, but some is basaltic lava, as at Sheepeater Cliffs.
35,000 ya: Oldest known cave art is created in Ardeche region of southern France.
More than 30,000 to 12,000 ya: Glaciers again cover Yellowstone intermittently: the Pinedale Glaciation, evident throughout the lower Lamar Canyon. Hot areas that will become geyser basins are under some 2000 feet (more than 0.5 km) of ice.
13,000 to 9,000 ya: Artifacts found in and near Yellowstone prove Paleo-Indian humans were here from the end of the last glaciation.
12,000 to 10,000 ya: Latest ice sheet retreats northward from midwestern and northeastern USA.
5,000 to 4,500 ya: Man begins to create written records (Sumerian language).
In the guidebook
In preparing the first edition of Yellowstone Treasures (2002), we decided that several items would have to be cut so as not to exceed 384 pages. The section Janet most hated to leave out was this geological time line. The sixth edition (2020) now has an updated geological time line on pages 297–98.
Copyright Janet Chapple. All Rights Reserved.