Twelve Millennia: Archaeology of the Upper Mississippi River Valley

Chapter Three

Past Climate of the Upper Mississippi River Valley The Pleistocene is a term given to that period of time between about 2 million and 11,000 years ago when the earth’s atmosphere cooled, permitting the development of huge amounts of glacial ice. During this Ice Age, portions of the earth’s Northern Hemisphere experienced several episodes of expansion and contraction of continental glaciation, particularly in areas of northern North America and western Europe. In North America, much of what is today Canada and the Midwest as far south as Missouri and Ohio were covered by a succession of glacial advances during at least four major periods. The ice advances were fed by compacting snow, forming glaciers that remained cool enough during the warm season that loss from melting and evaporation was less than the net annual gain. The main glaciers grew to several miles thick, deformed under their own weight, and spread out from their margins. The glacial margins acted as a combination plow and conveyer belt that incorporated sediment into the glacial ice. As they grew in size, glaciers shaped the land in a variety of ways, creating natural features such as c h a p t e r t h r e e the Great Lakes, the Upper Mississippi Valley, and the poorly drained lake districts of the northern Midwest. The expansion of continental glaciers in North America during the Pleistocene caused shifts in vegetation communities over vast areas. Drainage systems were redirected and forever changed. The tremendous weight of the ice sheets compressed the earth’s surface over which they spread. Even though glacial ice has been gone for thousands of years, in some portions of Canada the land continues to rebound about half an inch per year. Because the glaciers contained so much of the world’s freshwater, the sea levels of the world’s oceans were as much as 400 feet lower than today. The lower sea levels exposed large areas of the continental shelf, including the region between northeast Asia and Alaska now covered by the Bering Sea. The land connection between northeast Asia and Alaska that was exposed during the last glacial maximum is called Beringia (formerly the Bering Land Bridge). Beringia was a flat, grassy plain nearly 1,000 miles wide that was exposed for thousands of years. Neither northeast Asia nor western Alaska was covered by glacial ice, permitting the two-way passage of plants and animals. At the last peak of glaciation, about 18,000 years ago, the continental ice sheet spreading out from Hudson Bay merged with alpine glaciers extending out of the Rocky Mountains to block any overland passage between Beringia and the heart of North America. After about 14,000 years ago, these glacial masses separated as climate warmed, opening an ice-free corridor on the eastern slope of the Canadian Rockies. The best-known Pleistocene animals of North America are the extinct large land mammals, including the mammoth and the mastodon, relatives of the modern elephant. Other extinct animals include types of horses, camels, giant ground sloth, dire wolf, short-faced bear, and giant beaver, the latter of which reached weights of 400 pounds. These large Ice Age mammals are collectively called Pleistocene megafauna (fig. 3.1). The remains of the mammoths and mastodons are relatively abundant east of the Rocky Mountains, with thousands of specimens recovered from stream valleys, eroded areas, and construction sites. The finding of several unquestionable kill sites shows that human hunters encountered and pursued these animals at the end of the Pleistocene in many parts of North America. Nearly 20 finds of mammoths and mastodons are recorded in the Driftless Area (fig. 3.2), and one of these, the Boaz mastodon, appears to have been a kill. Our record of Pleistocene vegetation communities comes largely from the recovery of microscopic pollen grains from ancient deposits. Pollen grains collected from a stratigraphic sequence and extracted by special sampling 34 | p a s t c l i m a t e 3.1. Megafauna. Top: mastodon; left: long-horned Bison occidentalis; right: mammoth. Drawings by Laura Jankowski. 3.2. Example of megafauna find in the Driftless Area. Otto Swennes (left) and John Swennes (right) holding the upper end of the front leg bone of a mastodon found after a flash flood in Long Coulee, near Holmen. procedures are identified and counted to build a pollen profile that may...