Abstract

Sources of 1,852 basal till pebbles from sites in east-central New Hampshire were identified. Measurements of pebble size, shape, roundness, breakage, lithology, weathering, and distance from source were determined.

Roundness, breakage, and size distributions of the pebbles, when compared to their distance from source, indicate that a state of dynamic equilibrium is reached between the processes of abrasion and crushing. The average pebble survives long enough to abraid to a roundness of .5 on Krumbein's scale, then is crushed again. The early attainment of this equilibrium, within a mile of the source, suggests that the clasts go through at least several cycles of crushing and abrasion per mile of transport. After many miles of transport, certain pebble shapes gradually evolve as stable forms. Spheres are the most stable, discs and rods less so (in that order), and blades are least stable in the subglacial environment. Comparisons to experimental data for the same rock types suggest that abrasion and crushing are about equally effective mechanisms in the production of subglacial till. The pebbles appear to be deposited in multiples of 6 to 7 mi from their source.

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