Minnesota at a Glance Mapping Subsurface Sedimentary Rocks

Abstract

The two maps in Figure 1 are illustrations of the bedrock geology of Rice County. If both maps show the distribution of the same rocks, why are they so different? The geology has not changed in the nearly thirty years separating these two maps. It is our understanding of the geology that has changed. Prior to 1966, efforts to collect information on the subsurface geology were not organized and the distribution of data was spotty. Since the 1970 's, much greater emphasis has been placed on the systematic collection and preservation of data from water wells and test holes, although outcrops still provide much of our information about bedrock geology. Collection of subsurface geologic information is particularly important to bedrock mapping in Minnesota because much of the state is covered with thick deposits of glacial drift that hide most of the bedrock. This report outlines the data and methods used for mapping the relatively flat-lying sedimentary rock formations found in southeastern Minnesota. These rocks were deposited as sediments in oceans, and over millions of years became the solid rock formations we find today. Similar rock types are present in the northwestern comer of the state.