Radiocarbon dates from the Castle River Valley of southwestern Alberta indicate that the last major glacier advance down that valley reached its maximum extent 6200 radiocarbon years ago. This advance is considered to be late Pinedale in age. The Pinedale then ended with onset of the “Climatic Optimum” or “Altithermal,” which lasted in that region from about 6000 to 4500 years B.P.

The dates were obtained on bison bones found in outwash near the base of a 250-ft-high cliff. This cliff, here called Mountain Mill Bluff, lies on the south side of Castle River, 7 miles east of the Rocky Mountains and 6 miles due west of the town of Pincher Creek. The lower part of the outwash extends downvalley from a terminal moraine (Mountain Mill Moraine) of the former Castle Valley Glacier. The bison wandered into a meltwater stream draining from that glacier about 6200 years ago, drowned, and were buried by outwash accumulating below the moraine. As moraine and out-wash were laid down contemporaneously, dates on the bison bones indicate the time of construction of the moraine.

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