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Thickening Lake Ice Permits Sea Cave Visitation

Apostle Islands National Lakeshore

National Park News

One of the primary winter attractions at Apostle Islands National Lakeshore is a spectacular set of ice encrusted sea caves on Lake Superior’s mainland shoreline.   The surface of Lake Superior near the sea caves may be covered with ice for some or all of the period from mid-January to early April.  Thousands of visitors walk, ski, or snowshoe across the ice to explore the sea caves once the ice is in place.  On January 29th, park rangers determined that ice conditions allowed over-ice travel to these mainland sea caves.  More than 2500 visitors explored the caves during the recent President’s Day weekend.

Park staff go to great lengths to determine when ice conditions permit access to the sea caves.  Conditions are regularly viewed from the shore as the ice pack develops.  As the ice thickens, rangers cut holes in the ice near the sea caves to determine the quality and thickness of the ice.  MODIS satellite images ( help indicate the extent of the ice pack around the islands.  The lake ice must be thick enough and in place for several days before the public is notified that the sea caves are accessible.  Public notification includes updating the 24-hour “Apostle Islands Ice Line” at 715-779-3397 ext. 499, which provides visitors with current ice conditions at the mainland sea caves. 

Rangers monitor ice conditions until the spring breakup occurs.  The length of the ice season varies dramatically from year to year.  Some years the ice never thickens enough to permit access to the sea caves.  In other years, the ice sheet may last for up to two months.  When conditions allow access to the caves, park staff assume numerous responsibilities besides monitoring ice conditions.  Snow must be cleared from the access road, parking lot, walkways, and stairway.  Parking signs are posted and bulletin boards are updated with current orientation and safety information.  On busy weekends staff coordinates parking, explains fees, cleans restrooms, interprets sea cave geology, and responds to medical emergencies.  Walking on a frozen lake is an inherently dangerous thing to do.  Orienting visitors and making sure they are appropriately equipped to venture onto the ice is often the most important task during this exceptional winter event.  The goal is to make the visit memorable for all the right reasons.


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