Mother Lode, belt of gold-bearing quartz veins, central Calif., along the western foothills of the Sierra Nevada. The term is sometimes limited to a strip c.70 mi (110 km) long and from 1 to 61/2 mi (1.6-10.5 km) wide, running NW from Mariposa. Popularly it is used to mean the gold-bearing area E of the Sacramento and San Joaquin rivers and W of the Sierra Nevada. The discovery of alluvial gold on the South Fork of the American River led to the 1848 gold rush. Mark Twain and Bret Harte helped make the Mother Lode famous.