This one-day trip provides an overview of the hydrostratigraphic attributes of the Platteville aquitard in the Twin Cities Metropolitan area. As a shallowly buried, extensively fractured carbonate rock in an urban setting, vulnerable to contaminants, the Platteville has been the subject of a wide variety of geomechanical and hydrogeologic studies over the past few decades. This work, combined with our own borehole geophysics and outcrop observations, has led to a more comprehensive understanding of the Platteville. The field trip will provide examples of what we have learned from these many different data sources, which collectively lead to a characterization of the Platteville as a complex “hybrid” hydrogeologic unit. Under certain conditions, and from one perspective, it can serve as an important aquitard that limits vertical flow, whereas in other conditions, and from another perspective, it is best considered a karstic aquifer with bedding-plane parallel conduits of very high hydraulic conductivity that permit rapid flow of large volumes of water. One particular focus of the trip will be demonstration of what appears to be predictability in both vertical and bedding-plane fracture patterns that in turn provides some degree of predictability of flow paths in three dimensions. These relationships appear to be operative for the Platteville in other parts of the Upper Midwest where the Platteville is shallowly buried. We will demonstrate that effective management of such complex, karst, “hybrid,” hydrogeologic units requires a sophisticated, nuanced understanding of their heterogeneous behavior.