GENESIS OF BOSTON-HARBOR DRUMLINS, MASSACHUSETTS

Drumlins in Boston Harbor are composed of two superposed silty-sandy d iamicton units and subordinate sand and gravel. The lower, compact, un differentiated, diamicton is interpreted as a pre-Wisconsin till, base d on truncated weathering profiles preserved below an erosion surface. The upper, less compact, diamicton contains numerous, discontinuous a nd poorly sorted lenses and stringers of sand and sandy gravel that te nd to parallel the erosion surface. The upper diamicton and subordinat e sandy gravel lenses are interpreted to date from the late Wisconsin glaciation. Along the exposed sections of the Long Island drumlin comp lex, the preserved weathering profile on the lower diamicton is thicke r beneath the drumlin axes and is either thin or missing in the interd rumlin areas. This suggests that glacial erosion and streamlining of p re-Wisconsin deposits and their weathering profile took place after th e weathering profile developed and before subsequent deposition of the overlying diamicton unit. The latter drapes the erosional drumlin for m without contributing substantially to relief of the drumlin form.