A GPR survey images the effect of fluctuating lake levels in the Grand Island Tombolo’s internal architecture. Two progradational sequences are interpreted from the profile. The first progradational phase began when Lake Superior were lower and the tombolo was first exposed. This progradational phase is represented by shoreface progradation (radar facies 1) and strandplain development. The second progradational phase began as Lake Superior fell to its modern level from an intervening highstand. Shoreface progradation (radar facies 3) and strandplain development (radar facies 4) are again interpreted from GPR reflections. Both progradational episodes are marked by strandplain and shoreface reflections. However deposition during different environmental condition results in different shoreface reflection patterns.