Cosmogenic Cl-36 dating of the maximum limit of the Laurentide Ice Sheet in southwestern Alberta

Cosmogenic Cl-36 ages were determined on 11 glacial erratics from the summi ts of Porcupine Hills and Cloudy Ridge, Waterton valley, and the Foothills south of Cardston, Alberta. These erratics were derived from the Canadian S hield and the Rocky Mountains of the Waterton area. They were laid down by (1) the most extensive advance of a Canadian Shield centred continental ice sheet into this region (stratigraphically oldest glacial deposits); (2) a montane glacial advance from the Waterton valley (stratigraphically interme diate glacial deposits); and (3) an advance of continental glacial ice that overrode deposits of the intermediate-age montane advance. Zero erosion ra te Cl-36 ages of the erratics, uncorrected for snow cover, range between ab out 12 and 18 ka. They support the hypothesis that the Laurentide Ice Sheet reached farther into the southwestern Foothills than did all the previous continental ice sheets.