Fossils: the remains or impression of a prehistoric organism preserved in petrified form or as a mold or cast in rock. (Oxford)
Imagine...400 million years ago, before the present Rocky Mountains (USA) were up. They say it was a shallow,
calm sea that extended from present day Canada to the gulf. Corals are the shape of communities of little creatures.
Millions of tiny creatures over millions of years. The land rose up, the sea drained away and today, (where it hasn't been plowed),
you can find fossilized sea floor. The fossils are silica which is why they still exist today.
Fossils, Fossil Molds, Interior Casts
Identification Sources: The Audubon Society Field Guide to North American Fossils. Geological Survey of Wyoming- Paleontology of the Green River Formation. Bulletin 63 Dinosaur Valley State Park, Glen Rose Texas. Firefly Guide to Minerals Rocks and Fossils. Geology of the San Francisco Bay region. Geology of the Sierra Nevada.