Page:The Journal of geology (IA journalofgeology21894univers).pdf/98
The numerical expression of this example would be 20, -, 0: the minus sign not indicating subtraction, but merely signifying depression; and the zero indicating that no advance has been yet made in the new cycle. Another model exhibits a broad, gently undulating upland, traversed by a very narrow canyon. This interpreted to signify that an elevation occurred in the old age of the region, and that since then the streams have simply entered a new youth, incising young valleys in the uplifted peneplain. The formula of this example would be 60, +, 3. Examples involving deformation of a land surface, and the accompanying possibility of antecedent streams, are more complicated, and cannot be here introduced.
It is convenient to use the term, episode, for slight interruptions, so as to express their relative unimportance. I have also attempted the use of the term, chapter, for an unfinished cycle; but in talking with students this specialization of terms hardly seems necessary. Any region whose surface has been developed, partly with relation to one baselevel, and partly in relation to another; that is, any form whose development has involved two or more incomplete cycles, is said to have a composite topography. Many examples of such forms are encountered.
Special features of second or later cycles.—It is interesting to notice that, in certain cases, the adolescent stages of a second or later cycle, following the elevation of a region well advanced in a previous cycle, present features that did not characterize its first adolescence. One case of this kind is seen in meandering river gorges. Young rivers in their first cycle may cut crooked gorges, but they then follow consequent courses, and these cannot manifest the close relation between volume and radius of curvature that is seen in true meanders. This relation is found only in oldish rivers, which develop systematic meanders on their own flood plains. But if the region on which these rivers flow is introduced into a new cycle by uniform elevation, the rivers may cut down their meandering channels and produce meandering gorges. The Osage in Mis-