Ice sheets covered Minnesota as many as twenty times over the 2 million years of the Quaternary glaciation, each advance largely obliterating the results of the previous. Nearly all of Minnesota's glacial landforms derive from the last of them, the Wisconsin glaciation. One feature of this time in the park are the rock ridges near the campground, scoured smooth by the glacier and known as roches moutonnées. Another is the Thomson Moraine, a ridge along the northwest boundary. This terminal moraine comprises rock and debris dropped at the farthest reach of the Laurentide Ice Sheet's Superior Lobe.