Complex sentence: Geologists believe that the west boundary of this ancient seaway was the uplifted rocks of the Franciscan Assemblage that today make up the Coast Ranges, and the east boundary was a chain of ancient volcanoes located where the Sierra Nevada is today.
Split into simpler sentences: Geologists believe that the west boundary of this ancient seaway was made up of uplifted rocks of the Franciscan Assemblage that today make up the Coast Ranges. By contrast, the east boundary was a chain of ancient volcanoes located where the Sierra Nevada is today.