Fossils


When one views the Teton Range visions of vast, ancient seas do not usually come to mind. The peaks of the Tetons seem so powerful, so imposing, that it may be difficult to imagine this area as an almost featureless underwater plain. Both of these scenes, however, describe chapters in the geologic history of Grand Teton National Park. One of the most revealing clues for geologists unraveling the mystery of the Tetons’ past is the existence of fossils. Fossils are the mineralized remains or impressions of plants or animals from past geologic ages.

Fossils are typically found in sedimentary rock. One way that sedimentary rock forms is through the settling of suspended material (sand, gravel, or mud) from water. The settling material forms horizontal layers that thicken and lithify (harden to rock) over time. Sand, gravel, and mud are not the only material that settles to the bottom of aquatic bodies. Plant and animal material settles too. Once this organic material—perhaps the remains of an ancient fish—has settled, it is covered by the ongoing sedimentation process. In time the seas recede and the sedimentary layers are exposed to erosive forces such as wind, rain, and gravity. These forces break down the rock formations exposing successive underlying layers. Eventually, the fossil remains of a creature once buried under tons of sediment are exposed at the earth’s surface.

In the northern, southern, and, most dramatically, in the western portions of Grand Teton National Park are extensive formations of sedimentary deposits, some over a thousand feet thick. These formations contain the fossil remains of oceanic organisms. The presence of the fossils leads geologists to conclude that the area now occupied by the Tetons was once the floor of ancient seas. The seas were inhabited by algaes and corals, brachiopods (clamlike in appearance), and early ancestors of the crayfish—trilobites. Fossil records in Grand Teton date back to at least the Cambrian age approximately 500 million years ago.

Fossils do more than provide us with a fascinating look at prehistoric life forms. They are useful tools in dating geologic features, analyzing past climates, and tracing evolutionary processes. If you are fortunate enough to find a fossil during a visit to one of the national parks, please look but do not touch. Leave them to be rediscovered by the visitors and scientists of the future.

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