YOUNGER DRYAS LANDSCAPE CHANGE AND HUMAN OCCUPATION: A GEOARCHAEOLOGICAL AND ALLUVIAL RECORD FROM THE NORTHERN GREAT PLAINS AND ROCKY MOUNTAINS, ALBERTA, CANADA

There is increasing evidence that the Younger Dryas cool climatic episode (~11,000 to 10,000 ¹⁴C yr BP) brought considerable, though regionally varied, environmental changes across North America. Yet it has been suggested that human groups would not have been particularly challenged by the onset of YD cooling (Meltzer and Holliday, J. World Prehist., 23:1-41; 2010). Recognition that the Crowfoot Glacial Advance corresponded with the YD means that downstream stratigraphic responses can be expected in river valleys. Cooling would also have influenced montane mass-wasting processes such as debris flow activity on deglaciated slopes. This paper discusses evidence for both from the Bow River valley, Alberta, between Banff, in the Canadian Rocky Mountains, and Calgary, on the northern Great Plains. Stratigraphy at the Vermilion Lakes site, near Banff, records two pulses of debris-flow activity, one before 10,780 ¹⁴C yr BP and the other between ~10,300 and 9600 ¹⁴C yr BP, separated by a well-marked time of slope stability that corresponds with much of the YD. The valley-floor debris-flow fan surface was stable and saw repeated human occupation during the YD cool interval. Downstream in the Cochrane-Calgary area a thick, coarse alluvial fill (Bighill Creek Formation) was deposited between ~11,500 and 10,000 ¹⁴C yr BP, but dates on bone tend to form two groups: ~11,500-10,600 and 10,200-10,000 ¹⁴C yr BP. This is consistent with downstream redistribution of debris-flow sediments, with a mid-YD reduction or pause in aggradation. The gravels contain bones of now-extinct large vertebrates with southern, not Beringian affinities, suggesting a northward movement in relatively warmer pre-YD times, likely accompanied by Clovis hunters. People occupying intermontane valleys in the Banff area appear in fact to have benefited from YD cooling, to the extent that it increased the availability of alluvial fan surfaces for occupation. Warming at the close of the YD had the opposite effect, significantly restricting the availability of such areas.