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| | Chapter 28-Ecological Subregions of the United States |
 | | Bedrock is predominently Archean granite, metasediments, and greenstone; Cretaceous shale and minor Jurassic sandstone overlap the Archean on the western and southern margins. |
 | | It is characterized by moderately dissected, glaciated, flat to rolling plains that slope gently toward the Missouri and Mississippi River valleys, which bracket the Section on the west-south and east, respectively. |
 | | Bedrock beneath the drift is composed of lower Mississippian limestones, shales, and sandstones, well exposed on the uplands between the lower Illinois River and the Mississippi's flood plain, and in the bluffs overlooking the rivers. |
| www.fs.fed.us /land/pubs/ecoregions/ch28.html (4236 words) |
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