Complex sentence: Belleville, California was a gold mining boomtown in San Bernardino County that grew up rapidly following the discovery of gold by William F. Holcomb in Holcomb Valley in the San Bernardino Mountains early in 1860.
Split into simpler sentences: Belleville, California was a gold mining boomtown in the San Bernardino Mountains of San Bernardino County, California. It grew up rapidly following the discovery of gold by William F. Holcomb in Holcomb Valley early in 1860.