The Middle Proterozoic Midcontinent Rift System (MRS) of North America is a failed rift that formed in response to region-wide stresses about 1,100 Ma. In Iowa, the MRS is buried beneath 2,200-3,500 ft of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. An extremely large volume of sediments was deposited within basins associated with the rift at several stages during its development. Although the uplift of a rift-axial horst resulted in the erosional removal of most of these clastic rocks from the central region of the MRS in Iowa, thick sequences are preserved in a series of horst-bounding basins. Recent studies incorporating petrographic analysis, geophysical modeling, and other analytical procedures have led to the establishment of a preliminary stratigraphy for these clastic rocks and interpretations of basin geometries. This information has allowed the refinement of existing theories and history of MRS formation in Iowa. Additionally, drill samples previously interpreted as indicating the existence of early Paleozoic basins overlying the Proterozoic MRS basins were re-examined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels of the drillhole and were out of stratigraphic position. No deep Paleozoic basins exist in this area. These investigations led to the development of petrographic parameters useful in differentiating the Proterozoic MRS Red clastics from Paleozoic clastic rocks having similar lithologies.

Iowa's rock record is the product of more than three billion years of geological processes. The state endured multiple episodes of continental glaciation during the Pleistocene Ice Age, and the last glacier retreated from Iowa a mere (geologically speaking) twelve thousand years ago. Prior to that, dozens of seas came and went, leaving behind limestone beds with rich fossil records. Lush coal swamps, salty lagoons, briny basins, enormous alluvial plains, ancient rifts, and rugged Precambrian mountain belts all left their mark. In "Iowa's Geological Past, " Wayne Anderson gives us an up-to-date and well-informed account of the state's vast geological history from the Precambrian through the end of the Great Ice Age. Anderson takes us on a journey backward into time to explore Iowa's rock-and-sediment record. In the distant past, prehistoric Iowa was covered with shallow seas; coniferous forests flourished in areas beyond the continental glaciers; and a wide variety of animals existed, including mastodon, mammoth, musk ox, giant beaver, camel, and giant sloth. The presence of humans can be traced back to the Paleo-Indian interval, 9,500 to 7,500 years ago. Iowa in Paleozoic time experienced numerous coastal plain and shallow marine environments. Early in the Precambrian, Iowa was part of ancient mountain belts in which granite and other rocks were formed well below the earth's surface. The hills and valleys of the Hawkeye State are not everlasting when viewed from the perspective of geologic time. Overall, Iowa's geologic column records an extraordinary transformation over more than three billion years. Wayne Anderson's profusely illustrated volume provides a comprehensive and accessible survey of the state's remarkable geological past.

The 10th International Basement Tectonics Conference was conducted on the campus of the University of Minnesota, Duluth, in Duluth, Minnesota, USA, from August I through August 11, 1992. A total of 78 individuals were in attendance, 47 of which represented the host country, with the remaining 31 traveling from 11 different foreign countries. The four days of presentations were divided into three technical sessions, namely "Shear Zones", "Basement Control On Younger Structures", and "Rifting Midcontinent Rift System". This tripartite conference theme was also employed in the field trip agenda with three excursions being offered, all ably organized by Field Trip Chairman John C. Green. The pre-conference trip set the stage through a two day review of the "Archean and Early Proterozoic Rocks of Northeastern Minnesota". Under beautiful summer skies, 16 sites were visited within the Vermilion district of Minnesota, considered to be the best example of an Archean greenstone belt in the United States. All registrants participated in the mid-conference trip conducted along the gabbroic and volcanic terrain of the "Midcontinent Rift, Northeastern Minnesota".

Based on the Tenth International Basement Tectonics Conference held at the University of Minnesota-Duluth in August 1992, this volume contains 19 papers, 13 of which focus on the Middle Proterozoic Midcontinent Rift. An introductory essay discussing the Middle Proterozoic to Cambrian rifting in central North America is followed by contributions addressing topics including the Midcontinent Rift in Michigan and Minnesota, the Port Coldwell veins of northern Ontario, and petrography and sedimentation in the western Lake Superior region. The last five papers deal with the pre-Mount Simon basins of Ohio, the English Graben and the newly proposed East Continent Rift Complex, the Reelfoot Rift/Rough Creek Graben in the evolution of the Illinois Basin, and the A-type sheet granites in the Oklahoma Aulacogen of Cambrian age. Bandw illustrations. Annotation copyrighted by Book News, Inc., Portland, OR

Contains papers on cretaceous rocks in the northern Rocky Mountains, the Great Plains region, the Gulf Coastal Plain of eastern Alabama, and southwestern Minnesota; the Dakota formation; evolutionary and paleological implications of fossil plants from the lower cretaceous Cheyenne sandstone; and fau