Abstract Precambrian rocks in the Lake Superior region underlie all or parts of Minnesota, Wisconsin, and Michigan, an area along the southern margin of the Superior province of the Canadian Shield (Fig. 1). Except on the north, adjacent to Canada, the Precambrian rocks are overlapped by sedimentary strata of Paleozoic and Mesozoic age, which constitute a thin platform cover of relatively undisturbed rocks that thicken to the west, south, and east. Inliers of Precambrian rocks are exposed locally in southern Minnesota and Wisconsin, mainly in the flat valleys of major rivers, where erosion has cut below the Phanerozoic strata. The present landscape is subdued, and is inherited largely from Pleistocene continental glaciations, which produced a variety of erosional and depositional landforms. The glacier ice scoured the bedrock in the northern parts of the region, in much the same way as throughout most of Canada, and deposited materials of diverse lithology and provenance, as much as 200 m thick, over much of the remainder of the region. The Precambrian rocks in the region record an extended interval of crustal development and evolution that spans nearly 3 b.y. of earth history. This interval of geologic time is not continuously recorded in layered and intrusive units, but instead is punctuated by specific rock-forming and tectonic events that can be deduced from geologic relations and placed in a chronometric framework by isotopic dating. (Fig. 2, also see correlation chart for Precambrian rocks of the Lake Superior region, Morey and Van Schmus, 1986; and Bergstrom and Morey, 1985.)

Abstract The Seagull Lake-Gunflint Lake area along the Gunflint Trail (Cook County Highway 1) in T. 65N., R. 3 and R. 4W. in the northwestern part of Cook County, Minnesota (Fig. 1) is north of Grand Marais, the county seat and starting point for the Gunflint Trail. Although accessible by automobile or bus, much of the site is rugged and heavily wooded and requires considerable off-road travel. Except for obvious resorts and summer homes, the site is on public lands managed by the U.S. Forest Service. There is no single place where the entire geologic succession is laid out for easy study, so one must visit several localities to appreciate the significance of the area. This guide suggests 12 stops (Figure 2) that can be easily visited in one day and that collectively illustrate the major rock types and their stratigraphic and structural attributes. However, exposureis of such quantity and quality that independent exploration is heartily encouraged.

Abstract Jay Cooke State Park occupies nearly 15 mi 2 (38 km 2 ) in the northeastern corner of Carlton County in T.48N., R.15 and 16W. (Fig. 1). It is about midway between the village of Thomson and Fond du Lac, the westernmost suburb of Duluth. The park may be reached by following Minnesota 210 from the Carlton-Cromwell interchange with I-35 eastward for approximately 3.5 mi (6 km) through the town of Carlton to the St. Louis River. Immediately after crossing the river at Thomson, the highway turns sharply to the south and 0.1 mi (0.2 km) later enters Jay Cooke State Park (Fig. 2). The Park Headquarters, a picnic area, and an information building are located approximately 2 mi (3.4 km) intothe park. As with all Minnesota state parks, a daily fee or a yearly permit is required to use the park facilities. Minnesota 210 continues through the park along the north side of the St. Louis River for an additional 5.5 mi (9.3 km) where it joins Minnesota 23 in Fond du Lat.

Abstract Pipestone National Monument is located in parts of sections 1 and 2, T.106N., R.46W., north of the City of Pipestone, Pipestone County, southwestern Minnesota (Fig. 1). The monument is located 0.9 mi (1.5 km) northwest of the intersection of U.S. 75 and Minnesota 23. Proceed north on U.S. 75 for 0.4 mi (0.7 km); turn west and follow monument signs 0.5 mi (0.8 km) to the entrance. As with all national parks and monuments, no samples may be collected without a permit from the Department of Interior, U.S. National Park Service.

The Animikie basin in Minnesota developed during early Proterozoic time over and approximately parallel to the Great Lakes tectonic zone—an Archean suture between an ancient gneiss terrane to the south and a younger greenstone-granite terrane to the north. The evolution of the basin can be divided into an extensional stage with attendant deposition of stratified rocks, and a subsequent compressional stage assigned to the Penokean orogeny. The lower Proterozoic stratified rocks in the Animikie basin in Minnesota can be divided into two zones on the basis of pronounced differences in facies and thickness. These are (1) a relatively thin succession of predominantly sedimentary rocks north of the Great Lakes tectonic zone and (2) a much thicker succession of intercalated sedimentary and volcanic rocks to the south. The thicker succession is intensely deformed, metamorphosed, and intruded locally by igneous rocks. The close correspondence between the sedimentological and tectonic patterns implies that sedimentation and tectonism were both part of a tectonic continuum that began with the development of the depositional basin and culminated with the major tectono-thermal pulse of the Penokean orogeny. If a tectonic continuum is assumed, then any hypothesis to account for extension in the basin must also account for the compression. A close correspondence between sedimentological and tectonic patterns exists between the lower Proterozoic strata in Minnesota and those of the southeastern segment of the Animikie basin in Wisconsin and northern Michigan. Therefore, it seems likely that sedimentation in both segments occurred within similar tectonic regimes and that any hypothesis to account for the evolution of either segment must account for the evolution of both. The similarity of sedimentological attributes of lower Proterozoic strata in the Animikie basin and Phanerozoic strata deposited in geosynclines has prompted various plate-tectonic models to explain the formation of the basin. The structural features formed in the basin during the compressional stage, however, lack attributes typical of a consuming continental margin or colliding continental plates. The lack of these attributes does not negate the usefulness of plate-tectonic processes in interpreting the basin’s history, nor does it prove that such processes were not operative. We still do not know if the evolution of the Animikie basin followed the same “rules” of extension and compression that govern the evolution of a “typical” Phanerozoic geosyncline, or if the basin formed by a sequence of processes unique to its time and place.