The "Driftless Area" is generally known for its ancient geological landscape that was spared from the deposit of debris left behind by glaciers of the ice ages. The boundaries of the the Driftless Area are often debated, however.
The Wisconsin Geological and Natural History Survey has released a new Education Series publication, “The Driftless Area: The extent of unglaciated and similar terrains in Wisconsin, Illinois, Iowa, and Minnesota.” It is a four-page pamphlet with citations, plus GIS data, and a full-page size map of the Driftless Area. All of those items can be downloaded at https://wgnhs.wisc.edu/catalog/publication/000997.
The impetus for this publication was the proliferation of different boundaries to mark the Driftless Area, many radically different from the limit of glaciation but almost always identifying the Driftless Area (regardless of the boundaries used) as the unglaciated portion of the Midwest.
This work was done with co-authors from, and collaboration with, the state geological surveys of Minnesota, Iowa, and Illinois specifically so that it represents the unified statement from the four surveys on the matter.