The Jacobsville Sandstone is the uppermost and youngest layer comprising about of sandstone and conglomerate that underlies Lake Superior and fills the upper part of the Lake Superior segment of the Midcontinent Rift. These sedimentary strata overlie an additional of basaltic volcanic strata and mafic intrusions that fill the remainder of the Midcontinental Rift. A dangerous reef known as "Six Fathom Shoal" stretches more than north of the north point of the island, and is rumored to be the one the SS Edmund Fitzgerald shoaled on prior to sinking. A shallow reef to the southwest of Caribou Island Lighthouse lies only below the lake's surface. The now unmanned lighthouse which is owned by the Canadian Coast Guard is located on a tiny adjacent island called Lighthouse Island a few hundred feet across and positioned west of the southern tip of the island. When originally built, it was visible for and operated on a 30-second revolving cycle. These reefs are not coral reefs. Instead they consist of bedrock ridges and interfluves that lie between northerly trending bedrock valleys, which are known as "tunnel valleys." These tunnel valleys were excavated by subglacial meltwater at the base of the Laurentide Ice Sheet along pre-existing fractures and joints that exist within the bedrock floor of Lake Superior. Samples dredged from a shoal northwest of Caribou Island and close to one of these valleys resemble Jacobsville Sandstone. The lateral continuity and consistent and parallel direction of the tunnel valleys indicated that they are carved from friable sandstones that underlies the floor of most of eastern Lake Superior. There are six Caribou islands in Ontario, four of which are in Thunder Bay District. One of these Caribou Islands lies within Thunder Bay (the body of water) three km from Amethyst Harbour, Ontario and twelve km from Sleeping Giant Provincial Park at .