Guidebook 12. Field Trip Guidebook for the Precambrian Geology of East-Central Minnesota

Abstract

The bedrock geology of east-central Minnesota --generally bounded by latitudes 45 ON. to 47° N. and longitudes 92° 15' W. to 95° W. --is particularly interesting because the area contains a wide variety of igneous, metamorphic, and sedimentary rocks which span the entire range of Precambrian time. Unfortunately much of the bedrock is not well exposed. Those rock units that do crop out tend to occur as clusters 2 or 3 acres large or as strings 1 or 2 kilometers long; and these outcrop areas are widely separated by vast expanses of Pleistocene and Holocene materials as much as 140 thick. Therefore aeromagnetic and gravity data, and to a lesser extent, water-well and exploration records acquired over the years by the Minnesota and U.S. Geological Surveys were used to prepare a preliminary and somewhat generalized bedrock geologic map of east-central Minnesota at a scale of 1:500,000 (Morey, 1978).