The island measures 10,311 square kilometres (3,981 sq mi) in area, making it the 77th largest island in the world and Canada's 18th largest island. Cape Breton Island is composed mainly of rocky shores, rolling farmland, glacial valleys, barren headlands, mountains, woods and plateaus. Geological evidence suggests that at least part of the island was originally joined with present-day Scotland and Norway, now separated by millions of years of continental drift.
The northern portion of Cape Breton Island is dominated by the Cape Breton Highlands, commonly shortened to simply the "Highlands", which are an extension of the Appalachian mountain chain. The Highlands comprise the northern portions of Inverness and Victoria counties. In 1936 the federal government established the Cape Breton Highlands National Park covering 949 km2 (366 sq mi) across the northern third of the Highlands. The Cabot Trail scenic highway also encircles the coastal perimeter of the plateau.
Cape Breton Island's hydrological features include the Bras d'Or Lake system, a salt-water fjord at the heart of the island, and freshwater features including Lake Ainslie, the Margaree River system, and the Mira River. Innumerable smaller rivers and streams drain into the Bras d'Or Lake estuary and onto the Gulf of St. Lawrence and Atlantic coasts.
Cape Breton Island is joined to the mainland by the Canso Causeway, which was completed in 1955, enabling direct road and rail traffic to and from the island, but requiring marine traffic to pass through the Canso Canal at the eastern end of the causeway.
Cape Breton Island is divided into four counties: Cape Breton, Inverness, Richmond, and Victoria.