Imagine a sandy, tropical seashore extending across southern Minnesota-part of a vast, shallow sea that covers much of North America. The sandstone, shale and calcareous rock layers exposed across much of southeastern Minnesota are a geologic record of such conditions that existed hundreds of millions of years ago, during the Early Paleozoic. Although many people are not aware of the geologic history of the Paleozoic bedrock in Minnesota, the rocks are familiar to anyone who has traveled in southeastern Minnesota. The bluffs along the St. Croix, Minnesota and Mississippi rivers, and their tributaries, are composed of layers of Paleozoic-aged rock such as the St. Peter Sandstone and the Prairie du Chien Group. Paleozoic rocks lie beneath glacial deposits across much of southeastern Minnesota, from as far north as Taylors Falls, southwest to Mankato. They extend southward into Iowa and eastward into Wisconsin. To understand the detailed history of the bedrock in southeastern Minnesota, you need only be familiar with the processes of deposition and erosion. Paleozoic bedrock layers are sedimentary in origin. They are composed of particles of pre-existing rocks or minerals, or are precipitated by biogenic or chemical processes. Deposition is the accumulation of particles into layers, or beds. Small grains are dropped by wind or settle in water to form sandstone and shale. Elements such as calcium, magnesium, and iron, precipitate from seawater to form limestone or dolostone.
Short summary of Paleozoic sedimentary rocks in Minnesota. First published in 2000. Revised edition, 2002. 4 pages each.
Minnesota at a Glance Ancient Tropical Seas - Paleozoic History of Southeastern Minnesota..
Minnesota Geological Survey.
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