RHYTHMS m DENUDATION 763 



Pleistocene-Pliocene series, but for the upper and outer parts of the river 

 s_ystems which show the older stages the slopes are in general proportion- 

 ately flatter. 



We may tnrn next to a discussion of these relations of successive stages 

 of Valley cutting. On tracing the streams toward their sources, the in- 

 trenchment F from the last uplift is found to become shallower and 

 eventually disappear. Tn tlie progress of headward erosion the intrench- 

 ment from the last ii])lift has not had time as yet to reach to the upper 

 waters of the river. There the valle^y section E is higher, narrower, and 

 younger. In long streams, oi- those rising in the resistant terranes of 

 mountain regions, wliere the iuovement has been uplift witbout war])ing. 

 several of the latest stages of uplift may not yet have been felt. There 

 one stands in a Tertiary cycle of erosion, still in uninterrupted progress. 

 In order that the successive ])rofiles should he cut, t1ie youiigei' within the. 

 older, the oldest cycle, (\ must have been the longest : E the next longest, 

 and F the most brief. Tliis kind of a record implies an increasing: 

 rapidity of recurrence in diastrophism.^ 



This is a conclusion \^•itlL far-reaching implication which sbould be 

 tested carefully. Lei us examine, tlien, some other points of view. 



If the present baseh'Acl sbould endure for a suificient time, tbe streams 

 will sink to a Hatter grade, the valley slopes flatten down, and profile F 

 change to F', destroying the previous record; but since \a^c can not look 

 into the future, the nature of tbe series must be decided on tbe basis of 

 the record of the past. 1'hat riH-ord is shown in the relations of A, ('. 

 and E, and these indicat(^ an increasing ra])iditv in the recuri'ence of 

 uplifts. 



These uplifts need not. howex-ei'. lune been simple and free frnin short 

 period minor oscillations. As shown in broken lines in (igui'e 2, a short 

 pause, B, may have existed and yet bad the evidence of its existence total K 

 destroyed by a longer pause, C, and another, D, [)\- a longer jiause, l-]. 

 Even the development of peneplains does not i'(>([uire an agc-h)n!;' freedom 

 from. mov(^nients in liasi-lcvel. Alinor oscillations nuiv contiiuu,', luit thev 

 do not in that ease lead to a progressive.' rise of the land. In the lower 

 portions of the river systems alternate fil! and scour of the valley would 

 take place, but in the u])])er parts or on the interflu vial slopes erosion 

 would still continue and increase th.e ]j(M'fecti(m of the peneplain. 



This discussion, lejids ii< l)ack to the signifi(ance of the (piickening series 

 shown by tbe j)resent valb^v forms. The miickening is es])ecially in re- 

 gard to tlie Pleistocene movements. Foj- certain of those of Pliocene or - 

 older date no such bypotli(\^is of acceleration may be necessary, since each 

 new intrencbment Ik-Iow ;in old pene]j|ain Ix.' fore the previous cycle had 



