Natural arches are found along coastlines where wave action differentially erodes rock, leaving a roof of resistent rock over a softer layer. Along the volcanic rock coast underlain by the Lakes Shore Traps near Silver Island is a fine example of a lake arch.
Natural arches are found on Isle Royale (Suzy’s Cave, Caribou Island and Amygdaloid Island) and on the Keweenaw (here and at Bare Bluff). In the Portage Lake Volcanics more easily eroded lava flow tops are missing, while the flow bases above the missing tops are stronger and form the bridge that holds the arch in place. Wave energy is what forms the arch and what eventually destroys it. This arch is accessible by kayak in calm weather.
