34 
MUSEUM BULLETIN NO. 11. 
and erosion which act on its surface will be greatly stimulated. 
Following a definite course these agencies will proceed to dissect 
such a land mass. The larger streams will first deeply trench 
it with canyons; side cutting from the streams will skeletonize 
the interstream areas, that is, carve them into a number of peaks 
and narrow ridges which would slope down to the stream beds; 
and these ridges by decay and decrease of slopes will have slowly 
reduced their relief relative to the stream beds. This process 
will continue until the hills between streams have been cut 
down very low. As they sink lower and lower the forces of 
erosion, which vary greatly with the slopes upon which they 
work, will become very weak and finally a stage will be reached 
at which erosion is hardly perceptible and the land mass re- 
sembles a rolling plain with a few low residual hills upon it. 
The period of time in which this series of changes takes place 
is known as a geographic cycle 1 and has been divided into the 
stages of youth, maturity, and old age, while the final land form 
is called a peneplain. Youth is that stage in which the greater 
part of the down cutting of streams takes place. Maturity 
may be divided into two parts : early maturity in which the inter- 
stream areas are being skeletonized and the relief is greatest, 
the country being then decidedly mountainous; and late matur- 
ity, in which the reduction of relief is very rapid and the country 
is cut down from the rugged mountains to rounded hills. After 
that comes old age, in which further cutting down is slow. If 
old age be allowed to continue long enough without disturbance 
of the crust by either warping or change in level, a peneplain 
will finally result. During each of these stages certain char- 
acteristic forms appear, which are distinctive. Thus in youth 
the streams are ungraded, waterfalls and lakes often occur 
along their courses, and the slopes upon interstream areas are 
apt to be very different from those upon the main stream valley 
sides. In maturity stream beds assume smooth profiles, divides 
become ridges, the relief is most accentuated and then rapidly 
diminished, and differences in rock structures are brought out, 
^‘The Geographic Cycle,'* by W. M. Davis, The Geographic Journal, 
vol. 14,|l899, pp. 481-504. 
