Complex sentence: The town is divided by the Stillwater River, a branch of the Penobscot River, with part occupying part of Marsh Island, which is surrounded by the Penobscot and the Stillwater, and the other part on the mainland.
Split into simpler sentences: The town is divided by the Stillwater River, a branch of the Penobscot River. It occupies part of Marsh Island, which is surrounded by the Penobscot and the Stillwater, and the remainder of the town is on the mainland.