California's borders are bit strange for a state, but they're perfectly normal for a country. When the borders were decided, there weren't really any existing political boundaries to discuss except Mexico, so they went with geographical boundaries. The Sierras to the East, the Pacific to the West, the Cascades to the North, and Mexico to the South. The only place they really needed to smack a line on a map was in the middle of the Mojave desert, where they just continued the line from the Sierras to the Colorado River.