<span>Geologists have determined that about 1.1 billion years ago, the Earth’s crust began to split apart along a great rift zone now covered by Lake Superior. Huge volumes of lava flowed out onto the surface and cooled to form volcanic bedrock, mainly the dark type known as basalt. Several lava flows can be seen at the Upper, Middle, and Lower Falls and south of the Gooseberry River along the Lake Superior shore. The rifting also caused the flows to tilt gently toward the lake. These basalt lava flows, all along the North Shore, are also the birthplaces of Lake Superior agates.
