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1 ichigan is a geographically unique state, consisting of two large peninsulas that penetrate large inland freshwater seas separated from each other by only a few miles of water (see fig. 1).The physical and biological conditions of the two peninsulas differ enough to affect the composition of the fauna and flora of the two areas, yet the herpetofauna of the Upper Peninsula (UP) shares many species with both the northern Lower Peninsula and contiguous northernWisconsin. Geological History Understanding at least some of the geological history of Michigan is essential to understanding the distribution of herpetological species and their habitats in the state. The first thing to consider is the topography of Michigan, which is a product of both the underlying bedrock and the drastic effects of glaciation over the last two million years. Bedrock and the Great Lakes Over much of its surface, Michigan is a mass of unconsolidated Pleistocene (“Ice Age”) sediments, thousands of years old and ranging from about twentyfive to more than one hundred feet thick.These Pleistocene sediments consist mainly of boulders, cobbles, pebbles, sand, silt, and clay.They lie upon consolidated layers of ancient bedrock many million to more than a billion years old and thousands of feet thick. Michigan bedrock (see fig. 3) consists of many types of rocks representing the three main rock divisions: igneous, sedimentary, and metamorphic (see table 1). Common Michigan igneous rocks include ash, basalt, felsite, gabbro, granite, and obsidian. Familiar Michigan sedimentary rocks include sandstone, shale, siltstone, chert, flint, coal, gypsum, dolomite, limestone, and salt. Michigan metamorphic rocks include gneiss, marble, phyllite, schist, slate, and quartzite. Introduction Michigan as a Herpetological Habitat 1 M FIG. 1. Michigan, showing its counties and (1) Beaver, (2) Bois Blanc, and (3) Drummond Islands. Illustration by the author. FIG. 2. Geologic time scale. Illustration by Teresa Petersen. The Amphibians and Reptiles of Michigan 2 Unlike the situation in unglaciated southern Indiana and Ohio, outcrops of bedrock are rare in Lower Michigan and are mainly confined to road cuts and a few river and stream valleys. Some outcrops of bedrock occur in the eastern UP, especially in road cuts, and granitic outcrops are not rare in the western UP (see Dorr and Eschman 1970; Holman 1995a; Holman and Holman 2003). What does this mean to the herpetologist?The flaky or layered bedrock shales found in the meandering streambeds of unglaciated southern Indiana and Ohio are rare in Michigan. Here the streambeds consist of glacially derived boulders, cobbles, pebbles, sand, silt, and clay—a discomforting situation for people used to peering under flaky or layered shales for salamanders and small snakes. Moreover, the bedrock ledges, commonly found in the hillsides of southern Indiana and Ohio, occur in relatively few places in Michigan.When these ledges occur in southern Michigan, it is mainly where erosion has exposed bedrock in river valleys, such as has occurred in the Grand Ledge area west of Lansing. In the UP, occasional fissures and rock shelters harbor small amphibians (e.g., Holman et al. 2003) and perhaps reptiles.The rocky ledges, fissures, and rock shelters in the unglaciated regions in states south of Michigan provide not only temporary shelters for amphibians and reptiles but hibernating sites for several large species of snakes. But compared to states to the south, there is not much exposed bedrock under which Michigan herps might hide. The Great Lakes we know today are the products of relatively recent glacial ice, sedimentary debris, and meltwater acting on bedrock produced hundreds of millions of years ago during the Paleozoic era (see fig. 3). Before the Pleistocene, the basins that hold the present Great Lakes were stream valleys in the bedrock as well as products eroded from the bedrock in ancient times. When the ice sheets advanced, they tended to move along these valleys, following the path of the least resistant rocks. Ultimately, water from melting ice filled the gouged-out basins to form the Great Lakes. The present Great Lakes all drain in an easterly direction. Lake Superior drains into Lake Huron at Sault Ste. Marie, Lake Michigan into Lake Huron through the Straits of Mackinac; Lake Huron drains south along the St. Clair River into Lake Erie, and Lake Erie drains into Lake Ontario, which drains into the Atlantic Ocean by way of the St. Lawrence River. Lake Superior is the coldest and deepest, and Erie is the warmest and shallowest of the present lakes. The big lakes began to fill up in earnest as the...

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Additional Information

ISBN
9780814337134
Related ISBN
9780814332399
MARC Record
OCLC
794415464
Pages
320
Launched on MUSE
2012-07-18
Language
English
Open Access
No
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