The existence of an "ice-free corridor" through the ice sheets that once covered much of North America is not a dead issue despite some reports of its demise. The timing of a passageway south into the Great Plains is of considerable importance - and increasing disagreement - in first-Americans research. 

     Some believe it remained open throughout late glacial times, while others contend it did not open until the last Ice Age was almost over. Most, however, accept that it was closed, at least briefly. In my view, the best assessment probably is that of Canadian geologists Arthur Dyke and Victor Prest. They developed a series of maps showing, at 1,000-year intervals, the progress of deglaciation through the late Pleistocene and beyond.

       They show a narrow corridor- about 25 kilometers (15.5 miles) wide - opening up by 13,000 radiocarbon years ago (15,350 calendar years). The following millennium was likely the earliest period during which the ice-free corridor could be traversed, provided environmental conditions - muskeg (bogs and swamps), runoff, and wind-chill factors - allowed. A recently opened corridor would have been a formidable environment for people to penetrate, but eventually much of the soggy muskeg would have given way to grassland. 

     Between 12,000 and 11,000 years ago (14,000 - 13,020 cal BP), no significant barriers are apparent in the corridor. Recently deglaciated areas initially would have become wet-lands, but soon these areas would have changed into open prairies veined with rivers and lakes. 

     There is little doubt that the process of deglaciation was driven primarily by changes in global climate. Comparing the record of deglaciation and the limited archaeological record to ice-core records and data on Ice Age lakes produces patterns that are remarkably consistent across North America. 

Events that reflect global climate change typically are recorded (like chapters in the book of time) in geologic strata the distinct layers of sediment and soil you see along the side of a gully or roadcut. Climate change, in other words, drove geologic and hydrologic (water-driven) processes that left their mark in the earth. Those changes probably contributed significantly to the extinction of the Pleistocene megafauna (mammoths, sloths, saber-toothed tigers, and other beasts that disappeared with the time of Clovis). 

     This interpretation is based on geological and archaeological investigations at 47 sites excavated in the United States in the past 50 years. The stratigraphy seen at many of them can be reduced to a generalized, continental sequence. While every part of every site does not fit this model, enough do to convince me the stratigraphy is a direct response to climate change. 

     The sequence, with my generalized stratigraphic designations (images not shown), is as follows: 

     Alpha - 13,000 years ago (15,350 cal BP) and older: Strata reflect late Pleistocene lakes, springs, and other alluvial (river-borne) deposits. Excavations reveal the full complement of Ice Age animals (termed Rancholabrean fauna), but no generally accepted archaeological evidence. This would be the time of putative pre-Clovis people. The final part of this zone sees the shrinking lakes, reduced springs, and dwindling streams that accompany the end of deglaciation. 

     Beta 1 - 13,000 to 11,000 years ago(15,350 - 13,020 cal BP): By 13,000 years ago, water tables had fallen to their lowest levels in at least 15,000 years and streams grossly underfit the size of their full Ice Age valleys. Uplands became stable, but the sides of many valleys became eroded. Springs, lakes, and some streams dried up as water tables fell below the ground surface during what I have termed the "Clovis Drought." Deposits mostly were sand left in streambeds and other channels. Pleistocene megafauna became extinct during this phase, which occurred at the end of the warming period called the Allerod, when Clovis hunters seem to have been most active. The Clovis artifacts are found in association with the Rancholabrean megafauna, especially mammoths and large bison on a buried surface (landscape), at spring heads or along spring-fed streams. 

Beta 2a - 11,000 to 10,000 years ago (13,020 - 11,350 cal BP): The Allerod warm period ended with a sudden return to glacial climatic conditions - a harsh cold spell called the Younger Dryas. This period is recorded in the stratigraphy, most notably by a gray, black, or brown layer of organic pond or lake sediments (such as diatomite or peaty clay), wet-meadow soils, or black mats created by water-borne blue-green algae these reflect a rise of water tables that produced ponds or water-saturated lowlands. No in situ evidence of megafauna except bison appears in the dark mats, but Folsom, Goshen-Plainview, Agate Basin, and Hell Cap artifacts are found at many sites. 

Beta 2b - 10,000 to 7,000 years ago (11,350 - 7,810 cal BP): A brief episode of erosion is apparent at some sites between 10,000 and 9,000 years ago (11,350 - 10,185 cal RP). The transition from Paleoindian to Archaic lifestyles occurred during this period, but after the erosion, which was followed by a brief time of geological stability before a second major episode of erosion and deflation began. A notable feature of this sequence are the black mats of the Beta 2a period. No extinct Ice Age megafauna other than bison and no in situ Clovis artifacts have ever been found within the Younger Dryas deposits marked by the mats. No Folsom artifacts have ever been found in situ below it.

C. VANCE HAYNES, JR. is Regents Professor Emeritus at the University of Arizona in Tucson. He is a member of the National Academy of Sciences and was honored at the recent Clovis and Beyond conference in Santa Fe as New World Geoarchaeologist of the Century.