Yosemite Valley is a glacially carved valley in Yosemite National Park, located in the Sierra Nevada mountains of California. About eight miles (thirteen kilometers) long, it is surrounded by high granite cliffs and monoliths. Most of the park is also densely forested with several species of pine trees. Several streams and rivers flow into the valley and merge into the Merced River. Although the valley comprises about one percent of the total area of the park, it is where most visitors come. In the early Spring, the colorful shades of green in the valley provide contrast with the snow on the peaks. In the winter, snowfall can create dramatic images for photographers. In the late spring and early summer, the valley's elevation, combined with a clean atmosphere, create conditions for sometimes spectacular cloud effects. The fall colors come shortly after the first freeze, usually in October or November. Various scenes do more than simply change appearance with the seasons. Because of the high walls of the valley, an observer sitting at one place can watch the shadows shifting throughout the day, often dramatically. Changes in weather also bring about alternations to the landscape. Because the valley floor is just short of 4000 feet 1219 meters], mists and clouds often hang within the valley. Because of the valley's curvature along with its east-west orientation, the light within the valley is frequently dramatic at both dawn and dusk. If there are high clouds over the valley when the sun is just below the horizon in the west, light reflected from the bottom of the clouds can create an effect called alpenglow, which puts a soft pink glow on the granite. Half Dome is often bathed in pink as the sun is setting. The pictures in this volume illustrate views that change throughout the day as well as throughout the seasons.