An early settler from Florissant, Missouri named this area after his hometown. 34 to 35 million years ago, this valley was dominated by Lake Florissant, stretching 12 miles through an ancient forested valley and averaging 1 mile in width. Lush ferns and shrubs lived beneath redwoods, pines, cedars, and a mixed-hardwood forest. There were thousands of insects in the humid climate. Mollusks, fish, birds, and mammals lived in or around the lake. Again and again volcanic mud flows blanketed parts of the valley, one of them responsible for the creation of the lake. Now, each time the volcano erupts it showers the area with tons and tons of ash and pumice. Each rainfall washes the fine ash into the lake where it slowly covers the remains of creatures that have died and sunk to the bottom. Eventually these sediments become shale, transforming the buried remains into fossils.
Today, Florissant Fossil Beds National Monument preserves this internationally renowned site. More than 50,000 specimens have been collected by paleontologists working here. As so much of the lake life is preserved here, very little of the other forms of life that inhabited the area are preserved. Unless a bird or mammal died in or near the lake, its chances of preservation were very slim.