It also happened a few years back.
Extent of Glaciation in North America
Solid line: Approximate extent of glaciation during the last ice age.
Dotted line: Approximate extent of previous glaciation.
Above the black line, the area was covered with ice.
Below the line, people could have survived.
See other ice-age maps:
http://iceagenow.info/2011/05/ice-age-maps/
Thanks to Ron de Haan and Steven Goddard website for this hilarious insight.