On the Lake Superior shore of Wisconsin's Bayfield Peninsula, red sandstone cliffs are hollowed into arches, chambers and sea caves. In a hard winter, spray and seeping water freeze into curtains of icicles and columns of ice. When the lake freezes solid enough, visitors can walk across it to the mainland sea caves near Meyers Beach in Apostle Islands National Lakeshore.
The Apostle Islands mainland sea caves
Apostle Islands National Lakeshore was created on 26 September 1970 and covers about 281 km² of land and water. It takes in 21 islands, eight historic lighthouse towers on six of them, and a 12-mile strip of mainland shore. The mainland sea caves lie along that strip, a short drive from the park headquarters and visitor centre in Bayfield, Wisconsin. In summer, kayakers paddle along the cliffs. In winter, the same caves fill with ice.
A billion years of sandstone
The cliffs belong to the Bayfield Group of sandstones, laid down in the late Precambrian between almost a billion and about 660 million years ago. The middle layer, the Devils Island Formation, formed on sand flats sometimes covered by shallow water. Its thin, easily eroded beds are, in the National Park Service's words, "perfect for sea cave formation." Much later, Pleistocene glaciers shaped the islands, the last of them the Vanders Ice Lobe about 12,000 years ago. Since then, Lake Superior's waves have kept carving the softer layers into caves.
How the ice formations grow
The ice is a seasonal layer on top of ancient rock. Water seeps through cracks and pores in the sandstone and drips from ledges. Waves also throw spray against the cliffs before the lake freezes. In long spells of sub-zero weather, this water builds icicles that hang in rows from the cave roofs, thick ice curtains down the cliff faces, and columns where drips meet ice rising from the floor. The orange and red of the sandstone makes the white and blue ice stand out even more.
When the ice caves open, and how to visit
Walking to the caves means crossing miles of frozen Lake Superior, so the park opens the route only when strict conditions are met:
- long periods of sub-zero temperatures and low winds, so an unbroken ice shelf can form
- a shelf anchored to land at three points and reaching miles from shore
- thick, good-quality ice that can carry people and emergency vehicles, confirmed by auger tests
Those conditions are rare. Between 2000 and 2023, the caves were open on only about 2 percent of days. The main ice-cave winters were 2004, 2008, 2009 and 2014. The caves opened again for nine days in 2015 and for a single day in 2026. Walk out only when the park has announced the caves are open. Park staff update the ice line at (715) 779-3398, extension 3, when access is safe, and the NPS mainland caves winter page and current conditions and alerts give the latest status. Learn more in the National Park Service’s geology overview.
More Great Lakes ice
See the caves in an earlier winter in Apostle Islands Ice Caves: Brian Peterson’s 2014 Photo Gallery. Learn how Great Lakes ice volcanoes form, watch moving and stacking lake ice, or browse 24 Ice and Snow Formations: From Frozen Bubbles to Lake Michigan Lighthouses. For ice inside a glacier, visit the man-made tunnel inside Langjökull.
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