Fossil bones of zebras, beaver, otter, pelicans, and other water birds are found in sediments left from a 3,400,000 year old pond on the bluff across the river. Lava flows pouring out over the plains on this side met and dammed up sedimentary deposits washed in on the other side, making lakes and swamps. Here the river divides these two important geologic settings, formed at a time when the climate was wetter and the plains were tree-dotted grasslands where zebra-like horses used to graze.