Thirty-four million years ago, the Rocky Mountain region was much warmer, and the Florissant valley was forested with towering redwoods, false cypresses, pines, mixed hardwoods, and ferns, surrounded by drier slopes with scrublands, shrubs, and low trees. Streams flowed through the valley and provided additional moisture to the redwoods and riparian vegetation. The lobed, toothed leaf of the fossil maple, Acer florissantii, indicates warm-temperate growing conditions, while the smooth-edged Cape mahogany Trichilia florissantii with its extended drip tip is a leaf form common to wetter, subtropical climates. By thirty-three million years ago, the warm climate had cooled, and as conditions changed, the plants of the Eocene adapted, became extinct, or dispersed to other parts of the Earth. Fossil leaves serve as ancient thermometers for reading past temperatures, and by comparing fossil plants with modern floras, scientists estimate that the mean annual temperature at Florissant during the late Eocene was about fifty-six degrees Fahrenheit, similar to San Francisco today.