The Apostle Islands in Wisconsin are geologically rich and have caves formed by wave action, freezing, and thawing over centuries. The caves are in sandstone that was deposited in braided streams, sand flats, and shallow lakes during the late Precambrian era, between one billion and 660 million years ago. These sandstone units, known as the Bayfield Group, form the basement rock for the islands. The caves are sometimes called sea caves and sometimes ice caves because they can be both during the winter.
