Minnesota Geology Monday - Tombolo

Grand Marais is one of the best natural harbors on Minnesota's North Shore of Lake Superior.  The area was a natural attraction to European traders because of its use as a harbor.  The area has likely changed since those days, Grand Marais is French for 'great marsh', indicating the harbor was likely a marsh.

The harbor owes it's creation to the erosion and weathering of relatively soft lava flows that is found between more resistant flows.  One of the more resistant lava flows is/was an island that is now connected to the mainland by a gravel bar.  Wave action and currents in the lake built the gravel bar up to the point that it connected the land to the island providing a natural harbor.  The name for a connecting bar like this is a tombolo.