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. [Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXVIII. Chimney Rock. Dalles of the Wisconsin. Cross-bedding well shown in foreground near bottom.] [Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXIX. An Island in the Lower Dalles.] [Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXX. View in lower Dalles showing peculiar honeycomb weathering.] _The mounds and castle rocks._--In the vicinity of Camp Douglas and over a large area to the east, are still other striking topographic forms, which owe their origin to different conditions, though they were fashioned by the same forces. Here there are many "tower" or "castle" rocks, which rise to heights varying from 75 to 190 feet above the surrounding plain. They are remnants of beds which were once continuous over the low lands above which the hills now rise. In Plates XVII and XVIII the general character of these hills is shown. The rock of which they are composed is Potsdam sandstone, the same formation which underlies most of the area about Baraboo. The effect of the vertical joints and of horizontal layers of unequal hardness is well shown. Rains, winds, frosts, and roots are still working to compass the destruction of these picturesque hills, and the talus of sand bordering the "castle" is a reminder of the fate which awaits them. These hills are the more conspicuous and the more instructive since the plain out of which they rise is so flat. It is indeed one of the best examples of a base-level plain to be found on the continent.