Complex sentence: Gold prospecting in Calaveras County began in late 1848 with a camp founded by Henry and George Angel arrived in California as soldiers, serving under Colonel Frémont during the Mexican War.
Split into simpler sentences: Gold prospecting in Calaveras County began in late 1848 with a camp founded by Henry and George Angel. The brothers first arrived in California as soldiers, serving under Colonel Frémont during the Mexican War.