Why are the great lakes here? Their shape is closely related to glacial episodes and moraines left by the most recent retreat (Larson and Schaetzl, 2001).  But Lake Superior’s size and location is also coincident with the basin formed by the Keweenaw Rift, more than 1 billion years ago. Lakes and especially large ones are rare and do not survive very long.  The Great Lakes area is also marked by many smaller lakes, which is quite unlike other non-glaciated parts of the continent.