Complex sentence: In the 18th century, two Spanish expeditions came to the area; the second founded the Presidio of Santa Barbara and Mission to the east, and began the work of converting the Chumash to Roman Catholicism.
Split into simpler sentences: In the 18th century, two Spanish expeditions came to the area. The second founded the Presidio of Santa Barbara and Mission to the east, and began the work of converting the Chumash to Roman Catholicism.