Fig. 17. -- Sketch, looking northwest from Camp Douglas.
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CHARACTERISTICS OF VALLEYS AT VARIOUS STAGES OF DEVELOPMENT.

In the early stages of its development a depression made by erosion has steep lateral slopes, the exact character of which is determined by many considerations. Its normal cross-section is usually described as V-shaped (Fig. [18]). In the early stages

Fig. 18. -- Diagrammatic cross-section of a young valley.
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of its development, especially if in unconsolidated material, the slopes are normally convex inward. If cut in solid rock, the cross section may be the same, though many variations are likely to appear, due especially to the structure of the rock and to inequalities of hardness. If a stream be swift enough to carry off not only all the detritus descending from its slopes, but to abrade its bed effectively besides, a steep-sided gorge develops. If it becomes deep, it is a canyon. For the development of a canyon, the material of the walls must be such as is capable of standing at a high angle. A canyon always indicates that the down-cutting of a stream keeps well ahead of the widening.

Of young valleys in loose material (drift) there are many examples in the eastern portion of the area here described. Shallow canyons or gorges in rock are also found. The gorge of Skillett creek at and above the Pewit's nest about three miles southwest from Baraboo, the gorge of Dell creek two miles south of Kilbourn City, and the Dalles of the Wisconsin at Kilbourn City may serve as illustrations of this type of valley.

Fig. 19. -- Diagrammatic profile of a young valley.
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The profile of a valley at the stage of its development corresponding to the above section is represented diagrammatically by the curve a b in Fig. [19]. The sketch (Plate [XIX Fig. 1]) represents a bird's-eye view of a valley in the same stage of development.