Abstract

A total of 3234 unselected till pebbles and cobbles from 27 localities in central New York are of three lithologic types: sandstones, limestones, and shale (including siltstones). The most distant localities are about 20 miles south of the chief limestone outcrops at the Allegheny Plateau escarpment. Changes in size, shape, and roundness with increasing distance of glacial transport show a relative decrease of about 25 per cent in wedge-form and in rhombohedroid stones and a relative increase of more than 100 per cent in ovoids. Other forms show little variation. Although wedge forms tend to be more conspicuously striated than others, the inference is that rounding predominated over faceting, and that prolonged transportation by the middle-latitude ice sheets tended to produce stones of ovoid form rather than wedge shapes as has been claimed. All three lithologic types show these tendencies, but in different degrees depending on relative toughness. Reduction of the larger sizes provided a continuing supply of smaller ones, and gradual depletion of the larger sizes is reflected in the changing cobble-pebble ratio downcurrent from the source outcrops. Here also, rates of change are governed largely by relative resistance. However, with increasing distance of transport, cobbles increase in roundness more than pebbles, indicating less susceptibility to crushing and perhaps a faster rate of rounding. Only about 10 per cent of the till stones attained a degree of roundness greater than 0.4 on Krumbein's scale (c on Holmes's scale).

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