Paleontology and geology
The earlier Tertiary deposits occur in central and east central Idaho. These rocks are primarily volcanic, though some sedimentary rocks do exist. Only one fossil bone has been found in these rocks, but a number of fossil plants suggests that the climate was milder than it is today. The later Tertiary has a better fossil record across the state. The river and lake sediments of easternmost central Idaho contain fossils of fish, rodents, rabbits, horses, rhinos, camels, pronghorns, oreodonts (sheep-like mammals), and plants. Southern Idaho also has rich fossil plant and animal remains. The western Snake River Plain region was occupied by a series of lakes and tributaries whose deposits have yielded several hundred species of vertebrates, from fishes to mastodons, plus many molluscs and some plants. The most famous of these deposits can be seen in the Hagerman Fossil Beds National Monument, which includes the Horse Quarry. This site has yielded the fossil remains of about 200 zebra-like horses about 3.5 million years old. In northern and west central Idaho, thick layers of basalt are interbedded with ash and lake deposits that contain fossil plants. In the Clarkia Flora from this region, preservation is exceptional; some leaves even contain chlorophyll.