Complex sentence: It had over 12,000 head of cattle, one quarter slaughtered each year for the main exports of hides and tallow sent via river boats on the Petaluma River down to the San Francisco Bay.
Split into simpler sentences: It had over 12,000 head of cattle with about one quarter slaughtered each year. The cattle provided the ranch's main products-hides and tallow which were sent via river boats on the Petaluma River to the San Francisco Bay.