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.

The effect of differences in hardness on erosion is also shown on a larger scale and in other ways. Perhaps the most striking illustration is Stand rock (Plate [XXXI]), but most of the innumerable and picturesque irregularities on the rock walls are to be accounted for by such differences.

Minor valleys tributary to the Wisconsin, such as Witch's gulch and Cold Water canyon deserve mention, both because of their beauty, and because they illustrate a type of erosion at an early stage of valley development. In character they are comparable to the larger gorge to which they are tributary. In the downward cutting, which far exceeds the side wear in these tributary canyons, the water has excavated large bowl or jug-like forms. In Witch's gulch such forms are now being excavated. They are developed just below falls, where the water carrying debris, eddies, and the jugs or pot-holes are the result of the wear effected by the eddies. The "Devil's jug" and many similar hollows are thus explained.

WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXVIII.