<P> In North America , the ice covered essentially all of Canada and extended roughly to the Missouri and Ohio Rivers , and eastward to Manhattan . In addition to the large Cordilleran Ice Sheet in Canada and Montana , alpine glaciers advanced and ( in some locations ) ice caps covered much of the Rocky Mountains further south . Latitudinal gradients were so sharp that permafrost did not reach far south of the ice sheets except at high elevations . Glaciers forced the early human populations who had originally migrated from northeast Siberia into refugia , reshaping their genetic variation by mutation and drift . This phenomenon established the older haplogroups found among Native Americans , and later migrations are responsible for northern North American haplogroups . </P>
at its maximum extent continental glacial ice in north america extended as far south as
Missouri and Ohio Rivers