PROGRESSIVE WESTWARD EXPANSION OF NORTH AMERICAN CONTINENTAL ICE SHEETS DURING THE QUATERNARY AND IMPLICATIONS FOR THE TIMING OF INITIAL HUMAN OVERLAND MIGRATION INTO THE AMERICAS

There is extensive and robust stratigraphic and geomorphic evidence of progressive enlargement of North American (NA) continental ice sheets in a westerly direction during successive glaciations of the Quaternary Period. This culminated in a one-time coalescence of the Laurentide Ice sheet and valley glaciers from the Rocky and Mackenzie mountains and outlet glaciers from the Cordilleran Ice Sheet during marine isotope stage (MIS) 2. This singular coast-to-coast ice (CCI) event ended the pattern of broad ice-free corridors between Cordilleran and continental glaciers that was the norm during all previous Quaternary glacial maxima in North America. Recent discoveries of human settlements above the Arctic Circle in eastern Siberia during MIS 3 (~30 C¹⁴ ky BP) and an accumulation of archaeological sites in NA south of the limit of glaciation dating to MIS 3 (specifically <30 C¹⁴ ky BP to ~22 C¹⁴ ky BP) or contemporaneous with the CCI event during MIS 2 (specifically ~22 C¹⁴ ky BP to ~14 C¹⁴ ky BP) suggest that the limiting event for initial overland human migration into the Americas was the closing of the ice-free corridor rather than its opening as has been the orthodoxy.