The depth to which a drift sheet has been leached of its limestone depends on many factors, one of which is length of time. If other variables could be reduced, depth of leaching might be used as criterion of age. Porous gravel in flat-topped hills was measured, and carbonate content ascertained. Observations fall into a pattern of three belts: a southern belt leached 11–13 feet, a middle one leached 5–8 feet, and a northern belt leached 1–4 feet. Within each belt where the drift contains limestone, the depth of leaching is fairly constant even though percentage of carbonates varies considerably. The study therefore suggests that the depth of leaching of gravel is a fairly good criterion of age, and that the southern belt is Tazewell, the middle belt is Cary, and the northern belt is Mankato.

This would make the Olean of New York and the drifts of northeastern New Jersey and Long Island Tazewell, the Valley Heads, the Binghamton, and the “upper” drift of New England and coastal Gaspe Cary, and the drift south of Lake Ontario and north of the Adirondacks and southern Quebec Mankato.

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