The Illinoian till sheet here discussed was formed by the Illinois glacial lobe in connection with the maximum extension of that lobe. It seems quite well established that a lobe on the east, which covered southeastern Indiana and southwestern Ohio, and extended a short distance into Kentucky, also had its culmination at the Illinoian stage of glaciation. Farther east the Wisconsin sheet in many places reaches the glacial boundary, but there are small tracts of drift older than the Wisconsin, lying outside its limits in eastern Ohio, northwestern and northeastern Pennsylvania, and northern New Jersey, which may prove to be of Illinoian age, though this is as yet, not established.
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Frank Leverett (1898) studied this question.
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