During the last two million years, dramatic shifts in climate brought cold periods that lasted thousands of years, allowing snow to accumulate into ice sheets that covered most of Canada and the northern United States. The last great episode of the Ice Age, known as the Wisconsin Glaciation, ended about 10,000 years ago and radically altered this landscape as creeping ice excavated vast quantities of rock and soil, then left behind debris that formed a wide variety of glacial features. Ice Age National Scientific Reserve preserves evidence of these great glaciers in nine sites administered by the Wisconsin Department of Natural Resources in cooperation with the National Park Service, where present and future generations may study and enjoy this glacial inheritance. At Devils Lake, an ancient river once flowed through a deep gorge in the Baraboo Hills, but Ice Age glaciers dammed the gorge with glacial moraines to form Devils Lake. Today the Devils Lake Unit offers an excellent place to study the effects of continental glaciers, with hiking trails leading to rock formations and views of the lake, and recreational facilities there and on the North Shore resting on glacial moraines and outwash deposits.