The geologic story of the St. Croix National Scenic Riverway is rich. Evidence of the Midcontinent Rift and the Cambrian, Ordovician, and Pleistocene periods can be seen. The Midcontinent Rift occurred about one billion years ago when the North American continent began to split apart. Lava flowed from the rift and cooled, forming basalt, an igneous rock. Sedimentary rock, including Cambrian sandstones and Ordovican carbonate, was laid down when the area was covered by a vast inland sea 500 million years ago. The torrential outflow from Glacial Lake Duluth about 10,000 years ago at the end of the Pleistocene ice ages explosed the basalt formed during the Midcontinent Rift and the sandstones formed during the Cambrian and Ordovician periods. Potholes and morains were formed by the "retreat" of the glaciers.
