Dodge.] 260 [March 21, 
GKOGUArniCAi. Cyclic and Dkvki.opment of a Nokjial FJivkh. 
As all forms nssiiined l)_y waste in river valleys are intimately 
associated with the ueographic development of the accompanying 
river, I will now hrieHj' describe the developjnent of a river 
system and show the connection of alluvial terraces to some 
normal j)eriod of a river's life. 
The simplest conceivable river is one de\'elo])od naturally on 
a rising coastal plain or old lake bottom, now for some reason 
revealed. Such a river is called a constructional river, for its 
grade and course depend on the constructional slope of the new 
land. As has been described by Professor Davis, ^ a construc- 
tional stream would at once begin to develop a valley in the plain 
on which it is formed, in a manner to be described later on. If 
the land mass of which the j)lain were a part should continue 
stationary, the river would in the course of time reduce that land 
to the level of the sen, or to use a better term, to base-level. 
The time required to produce this effect is called a geographical 
cycle. It is very doubtful, however, if any continent has ever 
remaine<l stationary long 'enough for such a cycle to be com- 
pleted, 3^et still the cycle is a possible one. A river developed in 
such a simple cycle may well be taken as a type and called a 
normal river, and the topography resulting from the action of 
a normal river wouhl be simple l>ecause it would be formed in 
a single C3'cle. I will now trace briefly the history of such a 
normal river, following the stream from its birth on the construc- 
tional plain to its extreme old age when the constructional plain 
has been reduced to base-level. 
The first work of infancy of a river on a constructional j)lain 
would be to sink its channel into the rocks over which it flowed, 
beginning in its lower portion first. The river would thus gain 
an increased velocit}' on account of the greater grade and be able 
to work more rapidl3^ It w^ould then lengthen headwards l)y 
slowly gnawing backwards into the rocks of that region and 
would widen its channel by wearing laterally. The wasting- 
effect of the atmosphere would assist in the widening of the 
valley and in decreasiiig the slope of the valley sides as the walls 
1 The rivers and valleys of Pennsylvania: Nat. geog. mag., v. 1, no. 3. 
