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
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.