Split Rock Lighthouse State Park - Natural History - Geology

Geology

Much of the shore of Lake Superior is made of basalt erupted from the Midcontinent Rift System when the middle of the North American Plate began to crack 1.1 billion years ago. In a small tract that includes Split Rock Lighthouse State Park, additional magma intruded into the basalt and cooled underground into a harder rock called diabase. These flows also carried with them blocks of anorthosite, an even harder rock from the base of the Earth's crust, which became interspersed randomly in the diabase. Three large blocks of anorthosite form Corundum Point, Day Hill, and Stony Point beneath the lighthouse. In the southwest portion of the park another eruption formed a layer of red rhyolite. The rhyolite has eroded into several natural pillars, and forms the walls of the Split Rock River gorge.

Beginning 2 million years ago a series of glacial periods repeatedly covered the region with ice, scouring the bedrock and scooping out a great basin. The glaciers, and later meltwater, wore away less resistant rock, leaving behind hills and ridges of the harder diabase and anorthosite. At the end of the last glacial period the basin filled with meltwater. The water level fluctuated significantly over time due to blockages of the outlet and post-glacial rebound. The high-water Glacial Lake Duluth deposited clay sediments inland, while later, lower precursors of Lake Superior eroded bluffs and beach terraces.