_Effects of unequal hardness._--The process of erosion thus sketched would ultimately bring the surface of the land down to base-level, and in case the material of the land were homogeneous, the last points to be reduced would be those most remote from the axes of the streams doing the work of leveling. But if the material of the land were of unequal hardness, those parts which were hardest would resist the action of erosion most effectively. The areas of softer rock would be brought low, and the outcrops of hard rock (Plate XV) would constitute ridges during the later stages of an erosion cycle. If there were bodies of hard rock, such as the Baraboo quartzite, surrounded by sandstone, such as the Potsdam, the sandstone on either hand would be worn down much more readily than the quartzite, and in the course of degradation the latter would come to stand out prominently. The region in the vicinity of Devil's lake is in that stage of erosion in which the quartzite ridges are conspicuous (Plate XXXVII). The less resistant sandstone has been removed from about them, and erosion has not advanced so far since the isolation of the quartzite ridges as to greatly lower their crests. The harder strata are at a level where surface water can still work effectively, even though slowly, upon them, and in spite of their great resistance they will ultimately be brought down to the common level. It will be seen that, from the point of view of subaerial erosion, a base-level plain is the only land surface which is in a condition of approximate stability.
_Falls and rapids._--If in lowering its channel a stream crosses one layer of rock much harder than the next underlying, the deepening will go on more rapidly on the less resistant bed. Where the stream crosses from the harder to the less hard, the gradient is likely to become steep, and a rapids is formed. These conditions are suggested in Fig. 16 which represents the successive profiles (a b, a c, d e, f e, g e, and h e) of a stream crossing from a harder to a softer formation. Below the point a the stream is flowing over rock which is easily eroded, while above that point its course is over a harder formation. Just below a (profile a b) the gradient has become so steep that there are rapids. Under these conditions, erosion is rapid just beyond the crossing of the hard layer, and the gradient becomes higher and higher. When the steep slope of the rapids approaches verticality, the rapids become a _fall_ (profile a c).
