Complex sentence: The city is on the mainland of Texas and is connected to Mustang Island (which contains the city of Port Aransas) via an 11 mile-long causeway, and a free ferry that carries vehicles to the island.
Split into simpler sentences: The city is connected to Mustang Island (which contains the city of Port Aransas). The two cities are connected via an 11 mile-long causeway, and a free ferry that carries vehicles to the island.