COSMOGENIC ³⁶CL DATING OF QUATERNARY GLACIAL DEPOSITS IN SOUTHWESTERN NORTH DAKOTA

Scattered boulders and a discontinuous covering of glacial deposits indicate that areas southwest of the Missouri River in North Dakota were glaciated prior to, or during the Wisconsinan Stage. These deposits have traditionally been attributed to “early” glaciations. ³⁶Cl surface exposure dating was carried out on a suite of granitoid erratics located close to the maximum limit of presumed pre-Wisconsinan glaciation. Four of the six samples produced zero erosion ages between 27.8 and 28.8 ka, suggesting a very early Late Wisconsinan event. The dates are consistent with the commencement of expansion of Late Wisconsinan ice between 35 and 32 cal. ka. B.P. as suggested by recent ¹⁴C calibration data and δ¹⁸O records. This time frame implies that expansion of the Laurentide Ice Sheet into North Dakota was considerably more rapid and extensive than other depositional evidence suggests.