A peat deposit and submerged forest can be seen on the water side of Castalia marsh below the high water mark, and a fossil bed of shallow marine life is exposed in the Deep Cove Brook. The largest is Dark Harbour Pond. Smaller versions are present at Long Eddy Point, Indian Beach, Money Cove and other areas along the western side. Geological formations have given rise to some local names - White Head with its white quartzite head land, Seven Days Work, a cliff face with layers of lava flows pushed apart by igneous intrusions, the Bishop, a rock pinnacle on the northern end of the island, and Southern Cross on the southern end. Unfortunately, erosion has taken its toil on the Cross, only the base of the pinnacle remains.
Rock hounds can find agates, amethysts and zeolites with a little searching.
A copper mine operated at Sloop Cove for a few years beginning in 1872, with the copper shipped to England. The rock is white with yellow and blue veins. The Grand Manan Museum has a wonderful Geology section donated by the University of New Brunswick Geology Department.