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This map shows the extent of the Laurentide Ice Sheet (LIS) at
about 11,000 years before present, generalized from Deglaciation of
North America and overlaid on the North America shaded relief map
(courtesy of the U.S. Geological Survey EROS Data Center). Inset
shows the western limit of the LIS and the location of the Foothills
Erratics Train, which lies along a zone of coalescence between LIS
and glaciers from the Cordilleran Ice Sheet. Earliest known
archaeology sites are C - Charlie Lake; V - Vermilion Lakes; S -
Sibbald Creek; W - Wally's Beach; N - Niska. Image courtesy of
Lionel E. Jackson Jr.
Feature
The Ice-Free Corridor Revisited
Lionel E. Jackson Jr. and Michael C. Wilson
Editor's note: This feature is a history of studies of early Americans and serves as a primer for the following feature.
In 1927, a visit by eminent paleontologists to an
excavation in the wall of an arroyo near Folsom, N.M.,
confirmed that humans had coexisted with extinct
Pleistocene animals at about the time of the Wisconsin
Glaciation in North America (now known to have
occurred between about 25,000 and 10,000 years before
present). The excavation of a butchering site revealed
finely crafted thin, leaf-shaped stone projectile points
imbedded in the bones of large bison. These distinctive
projectile points, dubbed "Folsom points," were fluted
on each side to facilitate securing them to projectile
shafts.
Paleontologists uncovered an even older site with
indisputable evidence of human hunting of mammoths
in 1932 near Clovis, N.M. However, this location
revealed a different type of projectile point, longer than
the Folsom point and with a partial flute on each side.
The researchers named the points and the people who
created them "Clovis." The question immediately arose
as to how these hunters had reached the mid-latitudes of
North America at a time when the northern half of the
continent was apparently buried beneath the Laurentide
and Cordilleran ice sheets.
In 1933, W.A. Johnston of the Geological Survey of
Canada (GSC) proposed an answer. He and colleagues
had investigated the Pleistocene geology of the Rocky
Mountain foothills of Alberta between Calgary and the
Montana border. They believed that there had been
several overlapping advances of the Laurentide ice
sheets and glaciers from the Rocky Mountains. What
remained unclear to Johnston at the time, however, was
whether the glaciers actually had coalesced or whether
the mixed provenance tills along the Rocky Mountain
Foothills represented out-of-phase advances of glaciers
from the Rocky Mountains and the Laurentide Ice
Sheet. He favored a more restricted Wisconsin
Glaciation limit of the Laurentide Ice Sheet and, on this
basis, proposed that people could have reached the lands
south of the ice via a passage between glaciers
advancing into the plains from the Rocky Mountains
and continental glaciers.
In 1935, Ernst Antevs coined the term "ice-free corridor" to describe this passage. Johnston's suggestion was a
bold one, as it was based on very modest geological evidence. Such a corridor would have stretched 2,500
kilometers from the limits of glaciation along the Arctic coast in northern Yukon to northern Montana. Only the
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most general of geological surveys had been done for this area by 1933 and basic topographic mapping of much
of it was not completed for another 20 years. Systematic surficial geology mapping for most of the region was
not completed until the 1990s.
Despite numerous archaeological surveys, it has taken almost 60 years for a rigorous test of the hypothesis that
the ice-free corridor was the path by which the first Americans reached the mid-latitudes. To do so, researchers
have developed several analytical techniques, including radiocarbon and cosmogenic exposure dating, to resolve
geological and archaeological controversies. Through the investigation of pre-Clovis archaeological sites and
the reconstruction of North America's glacial past, scientists are now suggesting that the ice-free corridor may
not have been a feasible route for the early Americans.
Glacial reconstruction
The northern ice-free corridor encompasses the Mackenzie Mountains, the northern continuation of the Rocky
Mountains and the adjacent valley of the Mackenzie River. It is essentially a roadless area accessible only by
helicopter or airplane. Regional mapping of this area by GSC geologists Alejandra Duk-Rodkin and Owen
Hughes in the 1980s and 1990s determined that the limit of the Laurentide Ice Sheet from the last glacial
maximum represented the alltime limit of glacial ice cover. Furthermore, they cited evidence that suggested that
the maximum push of the ice sheet into the front (Canyon) ranges of the Mackenzie Mountains occurred as
early as 30,000 years before present as determined by radiocarbon dating (YBP).
However, a revisiting of the chronology of the maximum advance of the Laurentide Ice Sheet in this region by
Arthur Dyke of GSC and university colleagues in 2002 concluded that the maximum advance was more likely
in the range of about 21,000 YBP, which is compatible with the last glacial maximum. Duk-Rodkin also
mapped re-advance positions of the Laurentide Ice Sheet. She concluded that the actual opening of a corridor to
the south could not have occurred until about 13,000 YBP and extensive glacial lakes occupied much of the area
until around 12,000 YBP, affirmed in subsequent studies by Dyke and colleagues.
Settling the glacial history of the southern part of the ice-free corridor proved to be a thornier problem, with two
competing interpretations of stratified glacial tills along the Rocky Mountain foothills and adjacent interior
plains. One camp viewed these tills as representing deposits of several separate glaciations, spanning perhaps
hundreds of thousands of years. The other camp viewed them as the result of glacial advances and retreats
during a single glaciation, complicated stratigraphically through ice-thrusting of sediments. A significant aspect
of the controversy was determining if and when glaciers from the Rocky Mountains merged with the Laurentide
Ice Sheet. The key to solving the debate lay in the discovery of the origin of the Foothills Erratics Train, a
remarkable feature consisting of thousands of quartzite boulders ranging up to car-sized and even house-sized
blocks.
In the 1950s and 1960s, GSC geologist Archie Stalker and others traced the boulders over 600 kilometers from
northern Montana northwestward to the Mt. Edith Cavell area in Jasper National Park. They found that the
erratics train was as narrow as a kilometer in some areas and was found in a belt of mixed Rocky Mountain and
Laurentide Ice Sheet till. The only viable explanation for the narrow belt and its southward deflection parallel to
the Rocky Mountain foothills was that the erratics were transported along a zone of coalescence between valley
glaciers that advanced from the Rocky Mountains and the southwestern margin of the Laurentide Ice Sheet. But
when? The work of two groups would help settle the dating.
Jim Burns of the Provincial Museum of Alberta and
Robert Young, then at the University of Calgary,
obtained radiocarbon dates from bones found in gravels
beneath the single Laurentide till that exists in the
Edmonton area. This underlying preglacial gravel lacks
any clasts derived from the Canadian Shield, the source
of the Laurentide Ice Sheet. Twenty-nine sub-till
radiocarbon ages fell between 21,000 and 43,000 YBP.
These ages plus others from locations northwest and
south of Edmonton indicated that the last Laurentide
Ice Sheet was the only continental ice sheet ever to
inundate the Edmonton area and, by inference, reach
the eastern margin of the Cordillera to the west.
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This example of one of the Foothills erratics lies near Glenwoodville, Alberta. Its upper surface yielded a cosmogenic chlorine-36 exposure
age of about 17,000 years indicating that it was emplaced during the last glacial maximum. Image courtesy of Lionel E. Jackson Jr.
The second research group, comprising first author Jackson in collaboration with Fred Phillips at New Mexico
Tech and GSC colleagues, set out to directly date the emplacement of the Foothills Erratics Train and other
erratics, marking the alltime Laurentide Ice Sheet limit along the Canada-United States border, using
cosmogenic chlorine-36 exposure dating. Phillips had previously applied this technique to moraines in the
Mackenzie Mountains with Duk-Rodkin. Twelve exposure ages on the Foothills Erratics Train showed them to
have been emplaced between 11,000 and 18,000 YBP, which falls within the Wisconsin Glaciation.
Furthermore, 16 ages determined on erratics along the limit of glaciation by the Laurentide Ice Sheet yielded
compatible ages, in the range of 12,000 to 22,000 YBP. These ages not only corroborated the dated stratigraphy
from the Edmonton area, but also, taken with the results from the Mackenzie Mountains, showed that the last
Laurentide Ice Sheet surpassed all previous ones in extent in western Canada.
Artifacts and animals
The implications for the ice-free corridor hypothesis from this body of work were clear: There had been no ice-
free passage south along the eastern margin of the Cordillera from about 21,000 YBP (perhaps earlier) to as late
as 12,000 YBP. This left little or no time for Clovis people (circa 11,200 YBP) to reach the mid-continent; the
route would be untenable if people were in the Americas prior to 12,000 YBP.
For archaeologists, the glacial findings were disquieting; the idea that the ice-free corridor had been the route for
peopling of the mid-continent was well-entrenched and appeared to have no credible alternatives. However,
existing archaeological evidence from the corridor region is consistent with the new geological chronology. Few
archaeological or paleontological sites in the corridor area are older than 10,000 YBP, and only paleontological
sites have yet given dates over 11,000 YBP. The earliest archaeological sites include the Vermilion Lakes site,
near Banff, Alberta; the Sibbald Creek site in the Alberta Foothills; Charlie Lake Cave in northeastern British
Columbia; and Niska in southern Saskatchewan.
At Charlie Lake, people were using fluted projectile points more than 10,500 YBP, but these were points that fit
neither the Clovis nor the Folsom category; they are smaller, more triangular, and have multiple flutes at the
base, a distinctive enough pattern that they are now called Charlie Lake points.
One of the most striking relationships from a geographical standpoint is that the distribution of fluted projectile
points on the Canadian plains, as mapped by second author Wilson, fits very well into the southern half of a
corridor between the Cordilleran and Laurentide ice sheet margins for about 11,000 YBP as depicted in the
Deglaciation of North America by Dyke and GSC colleagues. This ice margin was reconstructed independently
based on nonarchaeological data. The oldest fluted-point sites are still those in the southern Great Plains and
American Southwest, so this distribution seems more consistent with a northward movement of fluted point-
using peoples from the mid-continent than with a southward movement through the ice-free corridor.
Archaeologists have found a few fluted points in Alaska, but none in dated contexts, giving little evidence to
suggest that they are comparably old. Cultural assemblages in Alaska that are of antiquity comparable to Clovis
are technologically different from those of the mid-continent.
Corroborative evidence for a northward movement of fauna into the opening corridor comes from the late
Wisconsinan vertebrate faunas (circa 11,500 to 10,000 YBP) found in the Canadian plains. These include
numerous finds, such as those from gravel pits in the Cochrane-to-Calgary and Peace River areas, described by
C.S. Churcher of the Royal Ontario Museum and Wilson, and from loess at the extensive Wally's Beach site on
St. Mary Reservoir in southern Alberta, described by Brian Kooyman, Len Hills and co-workers at the
University of Calgary. The faunal assemblage includes mammoths, camels, bison, helmeted musk-oxen and
horses that again strongly resemble southern species. Bison resembling Alaskan species did not appear on the
southern Alberta plains until about 10,500 YBP, if indeed even those were truly of Alaskan origin.
It appears that as the glaciers receded, southern-derived fauna moved northward to colonize the newly available
territory, which probably resembled open tundra to steppe for the first millennium or so of exposure. In fact, the
ice-free corridor also saw the transient presence of numerous, sometimes extensive glacially dammed lakes,
given that the regional gradient dropped eastward and meltwaters pooled against the retreating Laurentide ice
front, spilling southeastward along the margin in the southern corridor and northward in the northern corridor.
Ephemeral ice dams were prone to failure, resulting in frequent outburst floods along the ice margin.
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Despite decades of aggressive efforts by archaeologists from the universities of Alberta, Calgary and
Lethbridge, together with the "First Albertans" program of the Archaeological Survey of Alberta (now within
the Provincial Museum of Alberta), no credible pre-Clovis finds have yet been found in datable contexts within
the corridor area. A potentially exciting find of human remains, the "Taber Child," was made by Stalker of in
what appeared to be Pleistocene deposits, but was reinterpreted to be Holocene upon further excavations and
radiocarbon dating by Wilson and R.G. Forbis.
Archaeologists may ascribe part of the paucity of early archaeological material to widespread landscape
instability and preservation factors; however, the rarity even of radiocarbon-dated vertebrate remains older than
11,500 YBP stands in striking contrast to their abundance in immediately younger deposits. This evidence is
consistent with a view that the corridor was closed during the last glacial maximum and unavailable as a
dispersal route until Clovis times, if not later.
The weight of evidence therefore seems to point to the fact that the ice-free corridor was a transient feature at
best, of late Pleistocene age. The Cordilleran and Laurentide ice sheets coalesced, closing off any "corridor"
through most of the Wisconsin Glaciation. The opening of the corridor was biotic as well as physical: For each
species of plant or animal, there was an "opening" in terms of opportunity and dispersal ability. Physical
opening did not initiate a sudden stampede southward. Given the evident antiquity of South American Paleo-
Indian sites, which predate the oldest Clovis site by a thousand years, the role of the ice-free corridor seems
moot. Thus it would appear that evidence for the earliest human migrations to the mid-continent should be
sought elsewhere, and the west coast is becoming more and more credible as an alternative.
Dating differences
Radiocarbon dating is the most frequently used method of dating in the Quaternary. Carbon has two
stable isotopes, carbon-12 (12C) and carbon-13 (13C) — of which 12C is 500 hundred times more
abundant — and one unstable radioactive isotope, carbon-14 (14C), which forms in the upper
atmosphere, where its concentration relative to 12C is minute. Half of a quantity of 14C formed at any
given time will decay to nitrogen-14 every 5,730 years. Radiocarbon (14C) enters the tissues of all
living things, including bones and shells, while they are alive and consuming carbon from the
atmosphere or oceans. Once the organism is dead, the radiocarbon clock begins to tick. Because of its
geologically short half-life, the method is only applicable to approximately the past 50,000 years.
Radiocarbon dating measures the concentration of 14C in a specimen. Using that concentration relative
to the concentration of 14C present in the atmosphere in the year 1950 ("year zero" for 14C dating)
permits the calculation of an age based upon the 5,730-year half life. The method also assumes that the
14C:12C ratio in the biosphere and hydrosphere is in equilibrium with the atmosphere, and that the
organism has not incorporated additional 14C since its death.
However, by 14C dating of tree rings, each of which represent a calendar year (one revolution of Earth
around the Sun), researchers have determined that radiocarbon-dated years differ from calendar years.
This discrepancy is due to the fact that 14C concentrations vary over time because of fluxes in Earth's
magnetic field (and, more recently, nuclear weapons testing in the atmosphere). Much research has been
devoted to correction of radiocarbon years to calendar years because the relation is not a straight-line
function. Further, it is difficult to recalibrate radiocarbon dates older than approximately 15,000 years.
Some examples of conversions include:
14,000 radiocarbon years = 16,300 calendar years before present
12,000 radiocarbon years = 13,550 calendar years before present
11,500 radiocarbon years = 13,050 calendar years before present
11,000 radiocarbon years = 12,550 calendar years before present
To avoid misunderstanding and conversion error, scientists usually publish radiocarbon ages and define
the method used in citing ages throughout their publication: for example, 11,500 radiocarbon years
before present (YBP or RYBP). For a detailed explanation of radiocarbon dating, cosmogenic exposure
dating and other methods used for in Quaternary research, see Jay Stratton Noller, Janet M. Sowers,
William R. Lettis (Editors), 2000, "Quaternary Geochronology: Methods and Applications." AGU
Reference Shelf Series, Vol. 4, American Geophysical Union, Washington.
Renée Hetherington and Lionel Jackson
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Jackson is a Quaternary geologist with the Geological Survey of Canada in Vancouver, British Columbia, and an
adjunct professor of earth science at Simon Fraser University. He has mapped the surficial geology of extensive
areas of the Alberta Foothills and Yukon Territory. He has a longstanding interest in how and when the first people
came to the Americas.
Wilson is an instructor in both the department of geology (of which he is past chair) and the department of
anthropology at Douglas College, New Westminster, British Columbia. His primary research interests are vertebrate
paleontology, Quaternary stratigraphy, geoarchaeology and human-landscape interactions. He is also adjunct
professor of archaeology at Simon Fraser University.
Links:
"Quest for the Lost Land," Renée Hetherington et al., Geotimes, February 2004
Lionel Jackson's site on the Foothills erratics train
Further reading:
Dyke, A., Andrews, J.T., P.U, C., England, J., Miller, G., Shaw, J., and Veillette, J. 2002. The Laurentide and Innuitian ice
sheets during the Last Glacial Maximum. Quaternary Science Reviews, 21: (1-3): 9-33.
Fedje, D.W., J.M. White, M.C. Wilson, D.E. Nelson, J.S. Vogel, and J.E. Southon. 1995. Vermilion Lakes site: adaptations
and environments in the Canadian Rockies during the latest Pleistocene and early Holocene. American Antiquity, 60(1): 81-
108.
Fladmark, K.R., J.C. Driver, and D. Alexander. 1988. The Paleoindian component at Charlie Lake Cave (HbRf 39), British
Columbia. American Antiquity, 53:371-384.
Jackson, L. E., Jr., F. M. Phillips, K. Shimamura, E.C. Little. 1997. Cosmogenic 36Cl dating of the Foothills Erratics train,
Alberta, Canada. Geology, 25(3): 195-198.
Wilson, Michael C., and J.A. Burns. 1999. Searching for the earliest Canadians: wide corridors, narrow doorways, small
windows. In: R. Bonnichsen and K.L. Turnmire (eds.) Ice Age People of North America: Environments, Origins and
Adaptations, pp. 213-248. Oregon State University Press and Center for the Study of the First Americans, Corvallis, OR.
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