Across Atlantic Ice

by Dennis J. Stanford

  These ice fields, along with others around the world, captured so much of the earth’s available water that the sea level dropped an average of 130 meters. This exposed the continental shelves, creating land bridges that connected continents to continents and islands to continents, providing new lands and pathways to be exploited by both humans and animals.

  SOUTHWEST EUROPE IN THE LGM

  When the sea level dropped it revealed countless square kilometers of continental shelf around the world. In Europe, for instance, the French Aquitaine shoreline was extended more than 50 kilometers beyond the present coast of Europe, exposing approximately 20,000 square kilometers of low-lying, rolling limestone hills. Along the Cantabrian coast of Spain a narrow continental shelf extended the coastland as much as 10 kilometers farther out to sea. And like western Beringia and North America, the unglaciated southern portions of Ireland and England were joined to continental Europe by a vast land bridge (figure 5.3).

  This bridge was a low-lying plain with bogs and ponds, interrupted by a river system that drained northwestern France and southern Britain.13 Glacial silts and sands moved through this drainage system to be deposited into the sea at the rivers’ mouths. The sands that washed along the coastline were spread out along the beach and in some regions built dune barriers that provided areas of quiet waters, which allowed the development of estuarine and shallow sublittoral environments. Much of this sediment was eventually carried by strong winter winds back across Europe, covering a great deal of southwest France, known as the Landes. Dunes were common features along these shorelines; the Dune du Pilat near Bordeaux, the largest sand dune in Europe today, is a reminder of this ice age process.

  Karl Butzer has estimated that July temperatures in western Europe were as much as 10°C lower than they are today, drastically affecting the entire ecosystem.14 He has suggested that most of the interior of the Iberian Peninsula suffered significantly, as cold temperatures made even more severe by dry cyclonic winds blowing across the glaciers significantly lowered the productivity of the plains of Spain. Butzer also characterized the environment as an arctic steppe biome, where few plants grew and large herds of herbivores could not be sustained. As a result of these weather conditions, most animal and human populations moved to areas with more temperate climates. Pollen records and the increased number of archaeological sites indicate that more favorable conditions existed along the coastal areas of Spain, Portugal, and southwestern France and in the sheltered river bottoms of southwestern France.15

  Within 35 kilometers of the ice age shore, the Picos de Europa tower 2,500 meters above the north coast of Spain. The Picos and the narrow coastal plain are separated by a series of increasingly higher east-west trending mountain ranges. Each mountain range supported a unique ecological system with a diversity of natural resources. Along the coastline a succession of estuarine zones and river valleys led back into the piedmont, a narrow plateau, offering their own resources and providing relatively easy access and short travel distances to the uplands. Paleolithic peoples, perhaps operating from camps close to a variety of different ecological zones, could have exploited resources in each of these zones, taking advantage of seasonal concentrations of natural resources, such as game and berries.

  Butzer thinks northern Spain was a relatively treeless, steppe-like environment dominated in higher elevations by grasses and sagebrush. But because of its maritime character, the climate here would have been less cold and snowy than in the interior and less prone to bouts of severe weather. The ground cover at lower elevations was probably grasses and ferns, with heath plants in open windswept coastal regions. High counts of arboreal pollen suggest that locations sheltered from the wind protected small stands of trees, especially during warmer episodes. Pine, oak, and hazel trees may have grown on sheltered slopes with southerly exposures, and stands of alders along stream margins or floodplains.

  Spain’s high-mountain topography was probably mostly barren and snow-covered throughout the year, with many upper valleys glaciated. The tree line was around 1,600 meters high. At lower elevations, beech was the most common tree. Deciduous oaks extended up to 1,100 meters on relatively dry northern slopes. The Pyrenees were a formidable barrier between northern Spain and southwestern France. Its deep, ice-filled intermontane valleys would have provided few access routes, the most important of which may have been along the now submerged continental shelf.

  Beyond the Pyrenees the gently rolling countryside of the Aquitaine Basin gave way to the Landes region, where sands deposited during and after the LGM now cover the Paleolithic topography. In the central part of southwestern France the long, sinuous river valleys that snake around the hills of Dordogne dissect the Perigord region. These river valleys provided limestone caves and rock shelters that offered refuge to Solutrean bands, as well as numerous outcrops of readily chippable stones. The upland interior plateaus were more open landscapes with full tundra vegetation, but because the valleys were sheltered from the period’s full climatic rigors they preserved zones of protected forest.

  Bones found at archaeological sites indicate that the nonmigratory red deer was the animal most commonly hunted by the Solutrean people in northern Spain. Although generally regarded as woodland animals, red deer can survive in open grass-heath environments such as existed on the coastal plain of northern Spain. In fact, the bones of the Spanish red deer are relatively large, meaning that these animals made out quite well on the available forage. Horses and bison were hunted along the coast and in the broad river valleys. The remains of ibex and chamois, hunted on the mountain slopes, are less numerous, and there are only occasional traces of roe deer, boars, and various birds. Giant deer, woolly mammoths, steppe rhinos, musk oxen, steppe antelope, and bison are also insignificant in collections from Solutrean middens, although their remains have been dredged up from the submerged continental shelf, and they are depicted in cave art.

  Moving northward through the Basque country, reindeer become more common in faunal assemblages, and by southwestern France, where there were more snowfall and wet meadow habitats, reindeer replace red deer as the primary prey of Solutrean hunters. At the site of Solutré, just west of the Saône River in east-central France, an overwhelming amount of reindeer bones identify the Solutrean occupation as a typical specialization hunting site.16

  The extent of Solutrean use of the continental shelf is unknown at present. Its probable importance has been summarized by the archaeologists J. Rigaud and J. Simek: “Between one- and two-thirds of the land surface available in Late Pleistocene Aquitaine now lies buried either beneath the sea on the continental shelf or under the vast sand mantle of the Landes. . . . Models for Paleolithic demography, settlement systems, and subsistence that do not consider this full range of past environmental variability, including the now-buried regions, simply cannot be taken seriously.”17 This warning is equally valid for the Cantabrian continental shelf. Unfortunately, such models must be designed on the basis of indirect evidence and intuition and are therefore, like this study, informed speculation.

  There is no doubt, however, that Solutrean people used marine resources and were well aware of the richness of the seas. Shellfish remains collected from the extensive beaches and estuarine zones of the ice age coast have been found at several Solutrean sites. At La Riera Cave, for instance, nearly 10 kilometers inland from the beach, some Solutrean occupation levels contained abundant limpet and periwinkle remains.18 Salmon and trout have been found in faunal assemblages, as have a few seal bones. Since there is no contemporaneous evidence of boats, fishhooks, or harpoons, the seal bones are thought to have come from beached animals—but we argue otherwise in chapter 8.19

  A tremendous number of animals inhabited the Bay of Biscay whose remains have not been found in Solutrean archaeological levels. These include walrus, several varieties of whales, and six species of seals. The last were probably the easiest to capture and could have been one of the largest food sources for Paleolithic people on the coast of
northern Spain. Of the seal species likely to have been encountered in the LGM waters off the Vasco-Cantabrian coast, the hooded seal (Cystophora cristata) and the bearded seal (Erignatus barbatus) are migratory, whereas the ringed seal (Phoca hispeida), harp seal (P. groenlanica), gray seal (Halichoerus grypus), and common (or spotted, or harbor) seal (P. vitulina) are resident.20 Only the four resident seals have been found in the middens created by Late Stone Age Europeans.21

  How far inland should we expect to find evidence of marine exploitation? In other places where the ancient shoreline is now submerged, the answer is, not very far.22 Most Late Pleistocene sites now on or near the modern shoreline were 10–100 kilometers inland at the time they were occupied. Some have yielded the remains of fish and shellfish, but the quantities involved are very small.23

  In summary, the LGM climate of the north coast of Spain and southwestern France was much colder and drier than at present, but the relatively high insulation, ample winter daylight due to a relatively low latitude, and moderate snow cover favored rangeland productivity and supported a reasonable animal biomass. According to the geographer Karl Butzer, from a hunter’s perspective this was a very good environment, not a marginal one, with a diversity of dependable and concentrated resources.24

  NORTH AMERICA IN THE LGM

  On the other side of the Atlantic, the continental shelf of eastern North America is a huge geographic feature. It stretches along the Eastern Seaboard from the Grand Banks of northeast Canada southward to the Straits of Florida and in places reaches 400 kilometers beyond the present shoreline. Conservative calculations estimate that when the continental shelf was fully exposed during the LGM, it added approximately 445,000 square kilometers of land to the American continent (figure Intro.2).25

  Estimates of the amount of the shelf that was exposed by the lowered sea level have changed several times during the past fifty years.26 How much of it was covered by ice would determine how far north and west early sea travelers could have made landfall. The map we used shows that during the LGM many coastal stretches of the eastern Canadian Arctic remained ice free and provides evidence of ice advances extending farther out to sea to pre-LGM glaciers.27 This scenario would allow early sea travelers to have made summer landings on the shores of the Grand Banks.

  In this reconstruction the North Atlantic shelf was a low-lying coastal plain intersected by glacial ice lobes and various drainage channels, of which the Laurentian Channel would have been the most impressive. Lobes of the LGM Laurentide glaciation and local ice advances extended onto the continental shelf, but the piles of rocks left after the glacier melts—called terminal moraines if at the glacier’s front and lateral if at its side—indicate that they rarely reached the open sea. Rather, they ended behind a series of elongated topographic islands produced by terminal moraine remnants deposited by older Pleistocene glaciations.28 In many places pressure from the weight of the glaciers elevated the edge of the continental shelf above sea level. These forebulges created islands along the coast. Behind these islands the lobes of the LGM glaciers seasonally dissolved into ice shelves, meltwater pools, seawater channels, and bays. As deglaciation progressed, in a process known as transgression, rising seas swallowed up these landforms, which later became famous to fishermen as the Grand Banks, Browns Bank, and Georges Bank.

  The most recent analysis places the outer boundary of the glacial front from the outer edge of the Grand Banks southward to the northern end of the Georges Bank.29 If this reconstruction is correct, westward-bound Atlantic mariners could have made landfall on the eastern edge of the Grand Banks. This area may have been devoid of plant and animal life, but landfalls, no matter how barren the beaches on which they’re made, are of particular comfort to terrestrial creatures, especially conservative scientists.30 From there south to the Georges Bank are a series of islands, which dredging and paleoecological reconstructions suggest had a pine tundra mix of plants and mammoth fauna (figure 9.3).

  In any case, the continental shelf would have been above sea level and available for plant and animal colonization from Browns Bank, which fronted an ice-covered Gulf of Maine, southward to the Straights of Florida. Throughout this distance of some 1,800 kilometers, the shelf was transformed into a wide extension of the coastal plain. Off the coast of New Jersey the shelf is about 150 kilometers wide, while at Cape Hatteras it is 23 kilometers wide. At its greatest extent it reaches more than 130 kilometers east of the coast of central Georgia. From this point southward it narrows to less than 3 kilometers, at Key West. A series of sandy ridges up to 5 meters high along the entire East Coast shelf is thought to be LGM barrier beaches or beach dunes.31 (The subsequent sea transgression built post-LGM dunes.)32

  Today sea currents flow over the shelf and intermix with a multitude of freshwater drainage systems, well-developed estuaries, and large bays. By contrast, during the LGM primary rivers captured the second-order drainages and flowed together after breaching the coastal plain. Above Cape Hatteras the shelf slope becomes progressively steeper, producing increasingly higher banks extenuated by isostatic uplift above the LGM beach. The rivers that trended eastward across the outer shelf sculpted submarine canyons as they cut through the continental slope to reach sea level. These canyons would have been similar to small fjords, with relatively high banks flanking both sides of the river bottoms and extending up long, narrow inlet channels.

  FIGURE 9.3.

  North American Mid-Atlantic continental shelf at the Last Glacial Maximum, with finds of submerged late Pleistocene mastodons, mammoths, and artifact sites. (Based on Whitmore, Emery, Cooke, and Swift 1967.)

  Although seasonally ice choked, rivers north of the Susquehanna drainage system probably abraded and carried large sediment loads of glacial debris and loess. They also transported glacial meltwater. In spite of their high sediment content, they did not develop large deltas when they emptied into the sea, because heavier matter tumbled down the precipitous continental slope. Longshore currents carried finer sediment particles northward, where they were deposited on coastal dune ridges or simply intermixed with particulate matter settling to the bottom of the sea.33

  South of Cape Hatteras, the waters would have been saline because of low river runoff and the inflow of warmer, saltier waters brought by the Florida Current. The waters north of Cape Hatteras would have been relatively low in salinity due to river runoff and greater seasonal temperature variations. The summer offshore sea surface salinity during the LGM along the northern coast has been calculated to range between 30 and 32 parts per thousand, with sea surface temperatures of between 13°C and 16°C.34 This is about 10°C cooler than the salty waters of the current Chesapeake Bay. We suggest that traveling upstream through the canyons, one would pass rapidly through salt regimes to tidal mixing with freshwater channels. These environments would have been reasonably productive for various shellfish, and shells are common fossils in the LGM strandline deposits of Long Island and New Jersey. Fossil walrus bones are also common in this area, and it has been suggested that Long Island was an important walrus rookery.35

  The slope of the continental shelf ranges from 20 to 100 kilometers in width and has an average 4 degree drop in elevation. This would provide a suitable but narrow habitat of sea grasses for creatures that inhabit shallow offshore environments. Although planktonic foraminifers dominate the sediment of the entire continental slope, they increase from an average of 33 percent north of Cape Hatteras to nearly 90 percent of the carbonate sand fraction in the south.36 These abundances imply reasonable ecological productivity, especially in the south. This productivity is further indicated by shallow-water fossils such as mussels, oysters, coralline algae, oolitic beach rock, and coral reef debris and by freshwater and saltwater peat beds in relict sediments of littoral and sublittoral environments that existed during lower sea level stands. Fossils from gray whales and walrus are commonly dredged up from the continental slope. Their present-day feeding behavior suggests that these mammals turned over the slope sedi
ments, further enhancing the habitat’s productivity.

  The terrestrial environments of the continental shelf and adjacent Atlantic seaboard would have been horizontally zoned from north to south but not as well differentiated as the vegetation patterns along today’s coast. During the LGM some floral groups were displaced southward as much as 600 kilometers. Conditions such as geologic substrate, amount of available water, and seasonal storm tracks would have created a variety of local ecosystems. Many of these would have had plants and animals that do not coexist today; they were unique systems without modern analogues. The net effect would have been marked species diversity, with sustainable and predictable resources throughout each major zone.37

  To the north was a tundra environment where an open, spruce-dominated forest grew along the Laurentide Ice Sheet. This tundra snaked along the higher elevations of the Appalachian mountain chain as far south as South Carolina. The forest was identified by spruce, fir, and pine pollen peat dredged up from the northern side of Georges Bank.38 It is highly likely that these fossils represent local growth, for there are no upwind sources for the pollen. Thus, it appears that forests grew in sheltered localities even in the extreme north, directly fronting the glacier. An LGM peat sample from the submerged coastal plain on Maryland’s eastern shore was also dominated by spruce and pine pollen, although fir, oak, sedges, water lilies, ferns, and various moss species were also recorded. Other boreal trees—including fir, larch, hemlock, and birch—were much farther north than spruce. This evidence suggests that the prevailing forest ecosystem expanded to the margin of the continental shelf in various localities during the LGM.39