sentences:  The entire area is the raw southern edge of the Canadian Shield.Topsoils are thin and poor and their parent soils derived from the rock beneath or nearby rather than from glacial till, which is sparse.Many of this region's lakes are located in depressions formed by the differential erosion of tilted layers of bedded rock of the Canadian Shield; the crevasses thereby formed have filled with water to create many of the thousands of lakes and swamps of the Superior National Forest.