The Pewit's nest, about three miles southwest of Baraboo (m, Plate II), is another point of interest. Above the "nest," Skillett creek flows through a narrow and picturesque gorge in the Potsdam sandstone. The origin of this gorge is explained elsewhere. _Natural Bridge._--About two miles north and a little west of the village of Denzer (Sec. 17, T. 10 N., R. 5 E.), is a small natural bridge, which has resulted from the unequal weathering of the sandstone (see Plate XXVI). The "bridge" is curious, rather than beautiful or impressive. _The Dalles of the Wisconsin._--The _dalles_ is the term applied to a narrow canyon-like stretch of the Wisconsin valley seven miles in length, near Kilbourn City (see frontispiece). The depth of the gorge is from 50 to 100 feet. The part above the bridge at Kilbourn City is the "Upper dalles;" that below, the "Lower dalles." Within this stretch of the valley are perhaps the most picturesque features of the region. The sides of the gorge are nearly vertical much of the way, and at many points are so steep on both sides that landing would be impossible. Between these sandstone walls flows the deep and swift Wisconsin river. Such a rock gorge is in itself a thing of beauty, but in the dalles there are many minor features which enhance the charm of the whole. One of the features which deserves especial mention is the peculiar crenate form of the walls at the banks of the river. This is perhaps best seen in that part of the dalles known as the "Navy Yard." Plate XXVII. The sandstone is affected by a series of vertical cracks or joints. From weathering, the rock along these joints becomes softened, and the running water wears the softened rock at the joint planes more readily than other parts of its bank, and so develops a reëntrant at these points. Rain water descending to the river finds and follows the joint planes, and thus widens the cracks. As a result of stream and rain and weathering, deep reëntrant angles are produced. The projections between are rounded off so that the banks of the stream have assumed the crenate form shown in Plate XXVIII, and Frontispiece. When this process of weathering at the joints is carried sufficiently far, columns of rock become isolated, and stand out on the river bluffs as "chimneys" (Plate XXVIII). At a still later stage of development, decay of the rock along the joint planes may leave a large mass of rock completely isolated. "Steamboat rock" (Plate XII) and "Sugar bowl" (Plate XXIX) are examples of islands thus formed. The walls of sandstone weather in a peculiar manner at some points in the Lower dalles, as shown on Plate XXX. The little ridges stand out because they are harder and resist weathering better than the other parts. This is due in part at least to the presence of iron in the more resistant portions, cementing them more firmly. In the process of segregation, cementing materials are often distributed unequally.