The recognition of different cycles of erosion, separated by uplifts, is often easy. The principles involved are illustrated by Fig. 155 which represents an ideal profile of considerable length (say 50 miles). The points _a_, _a′_, and _a″_ reach a common level. Below them there are areas _b_, _b′_, and _b″_ which have a nearly common elevation, below which are the sharp valleys _d_, _d′_, and _d″_. The points _a_, _a′_, and _a″_ represent the cross-sections of ridges formed by the outcrops of layers of hard rock. If the crests of the ridges are level, the points _a_, _a′_, and _a″_ must represent remnants of an old base-level, _since at no time after a ridge of hard rock becomes deeply notched does it acquire an even crest, until it is base-leveled_.[60] At all earlier stages its crest is uneven. After the cycle represented by the remnants _a_, _a′_, and _a″_ was completed, the region suffered uplift. A new cycle represented by the plain _b_, _b′_, and _b″_ was well advanced, though not completed, when the region was again elevated, and the rejuvenated streams began to cut their valleys _d_, _d′_, and _d″_ in the plain of the previous incomplete cycle. The elevations, _c_ and _c′_ (intermediate in elevation between _a_, _a′_, and _a″_, and _b_, _b′_, and _b″_) may represent either remnants of the first base-level plain which were lowered, but not obliterated, while the plane _b, b′, b″_ was developing; or they may represent a cycle intermediate between that during which _a, a′, a″_ and _b, b′, b″_ were developed. If the intermediate elevations (_c, c′_) have a common height and level crests, the presumption would be in favor of the latter interpretation. If they be numerous and of varying heights, as is possible, they may in the field obscure the planes (_a, a′, a″_ and _b, b′, b″_) developed in the different cycles, which, in the figure, are distinct. [Illustration: +Fig.+ 155.—Diagram to illustrate cycles of erosion where the beds are tilted.] If the strata involved be horizontal the determination of cycles is sometimes less easy. Thus in Fig. 156, it is not possible to say whether _a_ and _a′_ represent remnants of an old base-level, or whether they represent the original surface from which degradation started. So, too, the various benches below _a_, such as _b_, _b′_, and _b″_ may readily be the result of the superior hardness of the beds at this level. For the determination of successive uplifts in the field it is necessary to consider areas of considerable size, and to eliminate the topographic effects of inequalities of hardness, and of certain other factors to be mentioned presently. [Illustration: +Fig.+ 156.—Diagram to illustrate cycles of erosion where the beds are horizontal.]