SL4-138-3843 (1 Jan. 1974) --- A part of northern California centered near San Francisco Bay photographed at 3 p.m. Jan. 2, 1974, from the Skylab space station in Earth orbit. This near vertical view encompasses the coastline from Monterey Bay (right) to about 50 miles north of Point Reyes (left) and includes, from bottom to top, San Francisco Bay (center), Sacramento Valley (left center), San Joaquin Valley (right center), and the snow-covered Sierra Nevada. Afternoon shadows sharply delineate a valley which parallels San Francisco Bay, crosses Point Reyes, and lies between the Bay and the Pacific coastline. This valley marks the location of the San Andreas Fault, a major break in the Earth's crust. Forces acting on the crust are causing the land west (bottom) of the fault line to move north relative to land on the east side. The Skylab 4 astronauts photographed major fault zones in South America, New Zealand, Japan and Africa for use in the study of worldwide tectonic system. Agricultural areas in the Sacramento and San Joaquin Valleys are indicated by the tan areas which are easily discerned in contrast to the green-gray background. Photo credit: NASA