AbstractDirect information on the isotopic composition and timing of the now-vanished Laurentide glacier can be found in subglacial calcite or in early diagenetic concretions from glaciolacustrine deposits. Th/U isotopic measurements on early diagenetic concretions frome Lake Deschaillons varves (St. Lawrence Lowland, Québec) date the early Wisconsinan major ice advance at ca. 80,000 yr (isotopic stage 5a/4 transition). Significant retreat occurred at ca. 46,000 yr (isotopic stage 3) as indicated by Th/U isochrones on early diagenetic concretions from Lake Gayhurst varves in the Appalachian foothills of Québec. During the maximum extension of the ice, which is dated at ca. 22,000 yr by Th/U measurements on subglacial calcite at Cantley (northern margin of the St. Lawrence Lowland), δ18O values (vs PDB) ranging from −16/−19‰ (surficial ice) to ca. −30‰ (basal ice) are inferred for the Laurentide ice isotopic composition. Superimposition of two phases of carbonate precipitation does not prevent the establishment of their respective age and isotopic composition through correlation of independent parameters and calculation of mixing lines.