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3 he idea that Michigan and much of northeastern North America were buried beneath a vast continental ice sheet a mile thick is difficult for most people to imagine. Yet the evidence for continental glaciation, compiled and corroborated by hundreds of scientists over the past 150 years, is irrefutable. Their work documents not only the complicated interplay of glacier advances and ice marginal retreats but also the growth and extinction of a multitude of vast glacial lakes that formed along the fluctuating ice margin as the deep depressions of the Great Lakes were uncovered. Understanding the lake sequence is crucial to a thorough appreciation of the region’s geologic history, yet it is exceedingly difficult to comprehend, even by the well initiated. Accordingly, this chapter will attempt to provide a cogent, although simplified, overview of present concepts regarding these ancient ancestors of the modern Great Lakes, giving special emphasis to the abandoned shoreline features observable in Pictured Rocks National Lakeshore. Chapters 4 and 5 then incorporate this information to provide a detailed review of Pleistocene and Holocene events in the region, completing the geologic history begun in chapter 2. INTRODUCTION Ever since the famous early promoter of the glacial theory, Louis Agassiz, first pondered the craggy outlines of Lake Superior more than a century ago, the abandoned shorelines of the Great Lakes region have intrigued geologists. Early investigators rightly attributed these features to periods of higher lake levels associated with a melting continental glacier during the Ice Age, but only recently has a coherent picture of the complicated series of glacial lakes and the strands they left behind begun to emerge. Relict (or “raised”) shorelines are common features along the Great Lakes. By “relict,” we mean landforms such as beach ridges, wave-cut bluffs, shoreline caves, ancient lakes and relict shorelines T 46 Chapter 3 sand spits, and stacks that are now abandoned and higher than the modern shoreline. Some of these features are now located well inland and can be traced for considerable distances. To the casual visitor, however, their formation and significance are difficult to grasp because of the enormous size of the lakes involved and our unfamiliarity with the mechanisms that formed them. Many visitors also confuse the large-scale changes in lake levels from the Ice Age with the much smaller variations caused by changes in temperature, precipitation, and runoff that characterize the modern Great Lakes from year to year. The latter rarely exceed 3 feet, whereas the former reach into the hundreds of feet. Simplifying the science is not easy, but by understanding just a few major principles, most of the intricacies can be easily understood. GETTING STARTED We’ll keep terminology to a minimum in this chapter, but a few terms are essential. For our purposes, basin refers to the elongated depressions in the Earth’s crust that hold the present Great Lakes. For example, when we refer to the Superior basin, we mean “the hole in the ground” (to put it perhaps too simply) that contains modern Lake Superior. The modern lake, however, is only the latest in a number of water bodies that have occupied this particular lowland. Because these lakes varied in size, elevation, configuration, and in the location of their outlets, they’ve been given different names, such as Lake Minong and Lake Duluth, yet all formed in the Lake Superior basin. Lake phase refers to the particular time period during which a lake existed. The Nipissing phase of the Great Lakes, for example, occurred between about 4,000 and 6,000 years ago. Such time-dependent terms allow geologists to distinguish between the lake itself and the time during which that water body existed. Geologists can determine the ages of glaciations and ancient shorelines using radiocarbon dating. This technique is based on the assumption that all living things contain a small proportion of the radioactive isotope 14C in their tissue. 14C is inherently unstable and will decay into the more stable by-product, 14N, at a known rate. This rate, called the half-life of 14C, is the time it takes for half of the 14C present to decay into 14N, about 5,568 years. When an organism is alive, the amount of 14C is continually replenished in tissue from the surrounding environment through respiration and intake of nutrients. However, upon death, the 14C begins to break down at a known rate, acting as a natural clock. If a lucky geologist later stumbles upon the preserved remains of the...

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