What is the name for routes on land that are exposed when Ocean levels fall during ice ages

Forty thousand years ago, during the Pleistocene,[1] two enormous ice sheets, the Cordilleran over the western Rocky to Coast mountains and the Laurentide over the lands to the east, covered much of Canada (Goebel et al. 2008:1498; see Figure 2.1). They retreated and expanded numerous times over the course of thousands of years. During the Last Glacial Maximum[2] (LGM) these ice sheets and others around the world grew in extent and depth that caused global sea levels to fall approximately 120 metres (Meltzer 2009:3). This happened because rain and snow froze into glacial ice instead of going into the oceans. This drop in ocean levels exposed the Bering Land Bridge, a landmass that was approximately 1,600 km wide (north to south) at its greatest extent and connected what is today Alaska and eastern Siberia at various times during the Pleistocene[3] ice ages. The Beringian environment was characterized by open grassland and tundra, ideal for foraging ice age animals.

Throughout the time of these ice sheet advances and retreats there were areas that were never glaciated, for instance in northern Yukon and Alaska. To understand when and how modern humans moved into the Americas and into Yukon and beyond, archaeologists and other researchers have attempted to study the routes they took. At least 20,000 years ago, and possibly earlier, ancestors of Indigenous Americans walked across the Bering Land Bridge and, at some point, paused in Beringia for a few thousand years. They then quickly migrated southward. This is called the Beringian Standstill model (Tamm et al. 2007). Whether the pause happened in western Beringia (Siberia) or eastern Beringia (Alaska) is still up for discussion; perhaps these ancient Beringians were a small, thinly spread population across Beringia. The discovery of three 11,500-year-old burials at Upward Sun River in Alaska, including that of infant Xach’itee’aanenh T’eede Gaay (Sunrise Girl-Child in Middle Tanana), and subsequent DNA testing indicate that the ancient Beringians, like Xach’itee’aanenh T’eede Gaay, had split from other Asian groups and become genetically distinct during the standstill period. About 4,000 years later the northern and southern branches of the Indigenous American family tree split again, creating two more genetically distinct groups, an Indigenous South American group and an Indigenous North American group (Moreno-Mayar et al. 2018).

Two possible routes of new world human colonization have been brought forward, popularly known as the Coastal Migration Route (CMR), and the Ice Free Corridor (IFC) route (Figure 2.1). The CMR posits that people followed a southward route along the west coast of Alaska and British Columbia, leaving evidence of their movements along the coastline (Easton 1992; Goebel et al. 2008:1499, 1501; Mackie et al. 2018). Previously, it was assumed that most of the coastline where sites might be found was now underwater and therefore unreachable for further study. There is now evidence that “the earliest known people to enter the Americas… likely arrived by traversing the Pacific coast using watercraft. This corridor would have been open for migration by 16,000 years ago as the Cordilleran Ice Sheet retreated and exposed tracks of land along the coast” (Waters et al. 2018:10). Recently, clear evidence of initial human coastal occupation was discovered in the form of human footprints “impressed into a 13,000-year-old paleosol beneath beach sands” at Calvert Island, British Columbia (McLaren et al. 2018). On Triquet Island, British Columbia, a 13,900-year-old hearth feature with associated artifacts has been found (Gauvreau and McLaren 2017). The Calvert and Triquest sites, and other coastal sites along the Pacific Coast, are demonstrating the very early use of a Pacific coastal human migration route at > 14,000 years ago (Mackie et al. 2018).

The Ice Free Corridor (IFC) route suggests that people may have moved from Asia into the Americas prior to the LGM, earlier than 20,000 years ago, by walking across the Bering Land Bridge and settling in unglaciated areas of eastern Beringia as early as 15,000 years ago (Morlan 2003). At the end of the LGM, the Laurentide and Cordilleran ice sheets retreated, opening an interior ice-free corridor on the southern side of the IFC at approximately 13,400 years ago. The northern (Beringia) side of the IFC opened up at approximately 13,000 years ago (Heintzman et al. 2016). This allowed large mammals, such as bison[4] and, it is argued, human populations that hunted them, to migrate northward from the ice sheets and then later southward through parts of Yukon, northeastern interior British Columbia, the Plains east of the Canadian Rockies, and into the rest of the Americas (Morlan 2003; Heintzman et al. 2016). Because the archaeological and phylogeographic[5]record indicates that the northern part of the IFC was only open after 13,000 years ago,  it “precludes the postglacial corridor as a southward route for initial human dispersal into the Americas, the corollary being that the first Indigenous peoples leaving Beringia probably took a coastal route…” (Heintzman et al. 2016:8061). The first peoples moving southward could not have occupied the rest of the Americas through the post-glacial IFC because it was not open in time. By the time the corridor was open people were already south of the ice sheets (Waters et al., 2018:1). Thus, people probably arrived south of the ice sheets following a coastal route, moving inland from the coast as they traveled to the southern hemisphere, reaching the “southern portion of South America by 14,200 years ago” (Waters et al. 2018:10).