Where and what is the Driftless?
The short answer
To a geologist the short answer is fairly clear. The Driftless is the area not glaciated during the Last Glacial Maximum, that is, during the most recent large scale ice age which ended about 11,000 years ago. Everything around the Driftless was covered by ice sheets at some point during the period, but the Driftless wasn't. This was recognized by geologists in the 1850s.
In this usage, drift means the sediment of all kinds that results from a glacier or ice sheet. From boulders to fine silt, the ice and melt water transport this drift from its original location and drop it wherever the ice melts or the water carries it. Originally, it was proposed that glaciers carried the "drift" while floating as icebergs, dropping them as they melted. That theory melted and was dropped, but the term was retained.
The labeled portion of southwest Wisconsin was not touched by the ice sheet. The Wisconsin and Mississippi valleys served as drainage for the sheets, which shaped them. The map does not include the small corner of Illinois which is in the Driftless proper.
In the attached map, the red areas show where the glaciers dropped their loads. The glaciated areas have moraines, drumlins, eskers, kames, outwash plains and more exotic formations, all shaped by the ice sheets.
The ice sheets passed the Driftless by, as shown on the embedded document below. Why did the various sheets bypass the region? The limestone bedrock may have drained away the water lubricating their underside. Despite the cold, the pressure of the ice melts its bottom, making it slippery like a wet floor.
Lake Superior, which is 1,300 feet deep, probably diverted the sheets. If Lake Superior were drained, its floor would be lower than Death Valley.
Note that the sheets never fully surrounded the Driftless at any time, but successive sheets extended as far as the Missouri and Ohio rivers.
But we never, ever do anything nice and easy
Unsurprisingly, millenniums of geology are not quite that simple. The topics below will be covered in more detail as this site develops.
There is a larger Paleozoic Plateau which extends into Iowa, Minnesota and Wisconsin. This was glaciated but not recently (in the last 100,000 years). The drift has mainly eroded and only traces remain. The resulting landscape looks and feels much like the Driftless, because it is the result of the same processes and underlying geology. The above document shows the Driftless under this larger definition. It about the size of West Virginia and twice that of Massachusetts.
There is Glacial Lake Wisconsin, labeled the Central Sand Plain in the map in the above document. It was not covered by the ice sheets, but submerged under its meltwaters. Sand washed into it for centuries, creating the sand counties that grow most the the world's cranberries. When it eventually drained, it helped shape the Lower Wisconsin River.
The Baraboo Range marks the edge of the Driftless, but their presence created Glacial Lake Wisconsin and impacted the Lower Wisconsin River Valley. A map in the above document shown it just below the Central Sand Plain.
The Wisconsin, Kickapoo and Mississippi Rivers and Military Ridge are part of the Driftless but deserve their own sections.