Calcareous concretions occurring on Grenvillian gneiss have been discovered north of Hull, Quebec. Their structure and isotopic composition (graphic; graphic; 14C age > 35 000 BP) indicate subglacial conditions of precipitation. It is concluded that they were deposited at the base of the Laurentide ice sheet. Assuming equilibrium conditions with the subglacial film of water during precipitation of calcite, it is possible to define a −27.5 to −31.8‰ (vs. "standard mean ocean water" (SMOW)) range for the oxygen-18 content of the ice.

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