The so-called Ice Free Corridor hypothesis has been the accepted human colonization route for the American continents since at least the 1930s. This route was postulated by archaeologists looking for a way by which humans could have entered North America during the late Wisconsinan ice age. Essentially, the hypothesis suggested that Clovis culture hunters arrived in North America chasing after megafauna (mammoth and bison) through a corridor between the ice slabs. The corridor was thought to have crossed what is now the provinces of Alberta and eastern British Columbia, between the Laurentide and Cordilleran ice masses.
Questioning the Ice Free Corridor
In the early 1980s, modern vertebrate paleontology and geology was applied to the question. Studies showed that various portions of the 'corridor' were blocked by ice from between 30,000 to at least 11,500 BP (i.e., during and for a long while after the Last Glacial Maximum). Since sites in Alberta are less than 11,000 years old, colonization of Alberta occurred from the south, and not along the so-called ice free corridor. Further doubts about the corridor began to arise in the late 1980s when sites older than 12,000 years (such as Monte Verde, Chile) began to be discovered; and clearly, people who lived at Monte Verde could not have used an ice free corridor to get there.
The oldest site known along the corridor is in northern British Columbia: Charlie Lake Cave, where the recovery of both southern bison bone and Clovis-like projectile points suggest that these colonists arrived from the south, and not from the north.
Clovis and the Ice Free Corridor
Recent archaeological studies in eastern Beringia, as well as detailed mapping of the route of the Ice Free Corridor, have led researchers to recognize that a passable opening between the ice sheets did exist beginning circa 14,000 cal BP (ca. 12,000 RCYBP). While too late to represent a passageway for preclovis peoples, the Ice Free Corridor, renamed the "western interior corridor" or "deglaciation corridor" most likely was the route taken by Clovis hunter-gatherers, as suggested by W.A. Johnson in the 1930s.
An alternative route of colonization has been proposed along the Pacific coast, which would have been ice-free and available for migration beginning about 14,500 BP. The change of path has also affected our understanding of the earliest colonists in the Americas: rather than Clovis 'big game hunters', the earliest Americans ("pre-Clovis") are now believed to have used a broad variety of food sources, including hunting, gathering, and fishing.
More details on the problems with the Ice Free Corridor hypothesis can be found in this article written in 2004 for Geotimes by Lionel E. Jackson Jr. and Michael C. Wilson.
Arnold, Thomas G. 2002 Radiocarbon Dates from the Ice-Free Corridor. Radiocarbon 44(2):437-454.
Burns, James A. 1996 Vertebrate paleontology and the alleged ice-free corridor: The meat of the matter. Quaternary International 32:107-112.
Dixon EJ. In press. Late Pleistocene colonization of North America from Northeast Asia: New insights from large-scale paleogeographic reconstructions. Quaternary International in press.
Mandryk, Carole A. S., Heiner Josenhans, Daryl W. Fedje, and Rolf W. Mathewes 2001 Late Quaternary paleoenvironments of Northwestern North America: implications for inland versus coastal migration routes. Quaternary Science Reviews 20(1-3):301-314.