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Jurassic Rocks - 208 to 146 million years ago

During the Jurassic, Pangea began to break apart. Africa broke away from North America, creating an ancestral Atlantic Ocean. North and South America rifted apart, forming the Gulf of Mexico. In western North America, the Rocky Mountains began to develop, and some of the oldest rocks found in the Sierra Nevada were deposited.

This period is well known for its dinosaur fossils. Many of the famous herbivores such as stegosaurus and brachiosaurus lived during the Jurassic. Much of interior North America was underwater, covered by the Sundance Sea. As sediment eroded from the mountain ranges on either side of the inland sea, the Sundance began to retreat. The sedimentary rocks that formed, especially those that make up the Morrison Formation, have yielded many dinosaur fossils. The end of the Jurassic is marked by an extinction that killed off many dinosaurs, including the very large herbivores.

Besides the Rockies and the Sierra, Jurassic rocks are also found at the center of the Michigan Basin.