Thirty-four to thirty-five million years ago, the Florissant valley was the site of Lake Florissant, a twelve-mile-long lake formed when volcanic mudflows dammed a stream that flowed south through the ancient valley. Fossil evidence shows a warm temperate climate during that time. Today, a much cooler and drier ecosystem with its own distinctive plant and animal life exists where ancient redwoods once stood. Climates change, plant and animal communities evolve, and the valley is still changing.