It is the northern of the two areas which in each case reveals the characteristic evidences of glaciation, while there is entire absence of such marks to the southward of the common border. Within the American glaciated region there is, however, an area surrounded like an island, and within this district (Fig. 324) none of the marks characteristic of glaciation are to be found. This area is usually referred to as the “driftless area”, and occupies portions of the states of Wisconsin, Illinois, Minnesota, and Iowa. Even better than the area to the southward of the Ohio and Missouri rivers, it permits of a comparison of the nonglaciated with the drift-covered region. [Illustration: FIG. 326.—“Stand Rock” near the “Dells” of the Wisconsin river, an unstable erosion remnant characteristic of the driftless area of North America (after Salisbury and Atwood).] =The “driftless area.”=—Within this district, then, we have preserved for our study a landscape which remains largely as it was before the several ice invasions had so profoundly transformed the general surface of the surrounding country. Speaking broadly, we may say that it represents an uplifted and in part dissected plain, which to the south and east particularly reveals the character of nearly mature river erosion (Fig. 177, p. 170). The rock surface is here everywhere mantled by decomposed and disintegrated rock residues of local origin. The soluble constituents of the rock, such as the carbonates, have been removed by the process of leaching, so that the clays no longer effervesce when treated with dilute mineral acid. Wherever favored by joints and by an alternation of harder and softer rock layers, picturesque unstable erosion remnants or “chimneys” may stand out in relief (Fig. 326). Furthermore, the driftless area is throughout perfectly drained—it is without lakes or swamps—since all valleys are characterized throughout by forward grades. The side valleys enter the main valleys as do the branches a tree trunk; in other words, the drainage is described as arborescent. In so far as any portions of a plane surface now remain in the landscape, they are found at the highest levels (plate 16 A). The topography is thus the result of a partial removal by erosion of an upland and may be described as _incised topography_. Nowhere within the area are there found rock masses foreign to the region, but all mantle rock is the weathered product of the underlying ledges.