The island represents part of an ancient volcano that erupted about 40 million years ago. The large, jagged rocks found throughout the island were formed by lava flows, and may be the oldest exposed rock in the Portland area. The island contains seven distinct habitats, including wetlands, forests and grasslands. A number of birds, including bald eagles and osprey, have been spotted in the area. The island is usually only accessible by a land bridge in the river during the summer when the water level is lowest.