The largest divisions of geologic time are eras, shown above in chronological order from the oldest on the bottom to the most recent on top. The scale at left shows the relative duration of each era. As the chart shows, geologists further divide time into periods and, in the Cenozoic Era, into epochs. The fossilization of animals in the Agate Springs area of Nebraska took place in the Miocene Epoch. Adjustments to this time chart are made as new data becomes available, so it should not be thought of as an unchanging reference. This diagram is adapted from one in The Emergence of Man series published by Time-Life Books.
| Geologic Time Chart | |||
|---|---|---|---|
| Period | Epoch | Time span (years before present) | |
| Cenozoic | Quaternary | Pleistocene | 10,000 to 2 million |
| Tertiary | Pliocene | 2 to 5 million | |
| Miocene | 5 to 23 million | ||
| Oligocene | 23 to 34 million | ||
| Eocene | 34 to 55 million | ||
| Paleocene | 55 to 65 million | ||
| Mesozoic | Cretaceous | 65 to 138 million | |
| Jurassic | 138 to 205 million | ||
| Triassic | 205 to 240 million | ||
| Paleozoic | Permian | 240 to 290 million | |
| Carboniferous | 290 to 365 million | ||
| Devonian | 365 to 410 million | ||
| Silurian | 410 to 435 million | ||
| Ordovician | 435 to 500 million | ||
| Cambrian | 500 to 570 million | ||
| Precambrian | 570 to 4,500+ million | ||
The Geologic History of Agate
Although the whole story of Agate Fossil Beds dates from the formation of the Earth four and one half billion years ago, only the last 600 million years is known in detail. It was about 600 million years ago that many plants and animals began to have hard parts—parts likely to be preserved as fossils. The few fossils contained in older rocks are often folded, twisted, squeezed, and distorted so that their original character is all but erased. That isn’t always the case, of course. Some of these old rocks, the Belt Series in Montana, look as though they were deposited only a few million years ago; they contain traces of algal colonies indicative of the generally simple life forms on the primitive Earth. The old rocks, deposited during the first four billion years of Earth’s history, record the Precambrian Eons, spanning eight-ninths of geologic time.
The evolutionary development of skeletal remains has aided in the study of geologic history. The last 600 million years have been divided into units for ease of discussion and comparison. The three largest divisions are the eras—Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). We are viewing all this from our vantage point at (what is now) the most recent episode of the Cenozoic.
To begin at the known beginning, we only need go back to the start of the Paleozoic Era some 600 million years ago. No Precambrian, Paleozoic, or Mesozoic rocks can be seen around Agate, but we know from rocks found in comparable areas about what to expect under the surface at Agate: thousands of meters of sedimentary rocks, most of them laid down in an ocean. During the Paleozoic and most of the Mesozoic eras, up until about 70 million years ago, the west-central United States was covered by seas. The area now occupied by the Rocky Mountains was then a long north-south trough in which thick sediments collected. To the east, the present Great Plains area was the floor of a shallower sea. Sediments collected in the trough and on the sea bottom. Gradually, over a period of 530 million years, the sediments accumulated to a thickness of over 2,100 meters (6,900 feet). A dynamic “give-and-take” process, this sedimentation was the result of periods when the sea rose and fell several times.
If the Paleozoic sounds a little dull, it’s because we haven’t told the whole story. During the Paleozoic Era, all the major groups of organisms evolved. The seas swarmed with trilobites and shellfish of all kinds, some weird and fantastic and some very like those alive today. With them coexisted fish and sea-lilies, seaweeds and giant swimming “scorpions.” For the first time, plants and then animals came out of the sea to live on the land. These events and creatures are preserved in Paleozoic rocks. The Mesozoic Era saw the development of mammals from reptiles, the rule of giant dinosaurs, and the beginnings of flight. Strange reptiles evolved and returned to the sea, or glided through the air on motionless wings. The Sundance Formation, deposited in the northern Rocky Mountains about the middle of the Mesozoic Era, is noted for the masses of bullet-shaped squid shells found in it. Water, land, and air were full of life.
In the Middle Mesozoic, the trough drained and much of it became an area of swamp and tropical forest extending from Montana to southern Utah. This was the domain of our largest dinosaurs, giant reptiles whose bones were preserved in impressive numbers. Como Bluff and Bone Cabin, Wyoming; Morrison, Colorado; and Dinosaur National Monument and Cleveland, Utah, are the sites of quarries where many fine specimens of Apatosaurus, Diplodocus, Stegosaurus, and Allosaurus have been collected. These caches of bones have made our large museums showplaces known throughout the world.
During the last 65 million years of the Mesozoic, the trough fluctuated up and down. During much of that time a broad, shallow sea covered central North America from the Gulf of Mexico to the Arctic Ocean. The sea was filled with fish like the giant Portheus (3.5 meters/11.5 feet long), with squid-like animals floating about in elaborate chambered shells, and with reptiles which had gone back to the water. Where there was a broad coastal plain, it swarmed with dinosaurs, the ones that make Lance Creek, Wyoming and Hell Creek, Montana famous collecting grounds for museum field parties. Yet at the end of the era the trough was a seaway again, and in the Agate area a final blanket of black mud, the Pierre Shale, was deposited in the sea.