Abstract

Structures visible in a recent photograph of a clastic dike protruding downward from 'till' into sand at St. Ludger, Quebec suggest that it may not be till as previously reported, but part of a layer of stony lake clay deposited beneath floating ice. Flame structures and faults indicate that the underlying sand changed from a plastic to an elastic state. Probably a layer of stony lake clay was drawn down into the underlying sand by reduced pore pressure developed in part of a closed system. This occurred when adjacent sand in the system was dilated by the shear stress resulting when floating glacier ice grounded. Glacial thrusting may have provided further intrusive impulse after the initial downward deflection.

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