492 H. E. Gregory — Formation and Distribution of 



period during which the relief is increasingly faint. At the 

 beginning of a cycle the features of youth in a landscape may 

 be superposed upon features indicating adolescence, maturity, 

 or old age. But whatever the initial topographic expression, 

 streams of youth start at the long and arduous task of establish- 

 ing and maintaining grade. If youth follows maturity, gravel 

 is ready at hand ; if youth follows old age, gravel must be pre- 

 pared and furnished to the stream before distribution can be 

 effected. The production of gravel is an accompaniment of 

 vigorous erosion and accordingly its proportion in total land 

 waste is greatest in youth and early maturity. Moreover 

 gravels prepared during youth normally contain a large amount 

 of undecomposed material since rapid down-cutting of channels 

 favors mechanical agents of erosion. 



At the beginning of a cycle, the stream finds itself out of 

 adjustment and at once undertakes to remedy this defect. If 

 the initial stream profile is over-flat, grade is established by 

 building steeper slopes beginning at headwaters and, until a 

 profile suitable for carrying gravel with a given volume of 

 water is developed, gravel accumulates in the upper parts of a 

 stream course. If the profile at the beginning of a cycle is over- 

 steep, coarse w r aste is distributed along the valley to depths 

 required to bridge the concavities and establish grade. If young 

 and mature stages of the stream's history be of long duration, 

 gravel may thus accumulate to great thickness. With fluctuat- 

 ing stream volume in response to seasonal or cyclical flooding, 

 gravels are carried farther and farther down valley. In matur- 

 ity streams are competent to carry the load furnished ; i. e., 

 grade has been established. But probably no stream is in a 

 graded condition throughout its entire course, and the adjusted 

 profile of short stretches changes frequently and at times 

 abruptly. With the continually changing conditions of volume 

 discharge, grade is maintained by shifting the load laterally 

 and distally but with net movement downstream. It is import- 

 ant to bear in mind that pebbles are carried in one direction by 

 rivers, whereas waves and wind transport sediment back and 

 forth. On the gentle slopes of old age the maximum amount 

 of material is carried in solution and gravels are normally 

 absent. 



The arrangement of gravel deposits on the stream bed may 

 be observed at stages of low water. As displayed on the floor 

 of certain ephemeral streams on the Navajo Reservation, the 

 masses of gravel are irregular in distribution and yet conform 

 to a poorly defined pattern. For stretches of a few tens, rarely 

 of a few hundreds feet, the entire bed is paved with pebbles 

 or cobbles. More commonly the deposits have the form of 

 lenses 10 to 100 feet long, 3 to 20 feet wide, with gentle slope 



