136 PRINCIPLES OF STRATIGRAPHY 



only temporary, for as soon as the supply ceases the river will re- 

 vive and begin active cutting again. This may be compared to a 

 temporary depression or weakening from overwork, which pro- 

 duces temporarily a weariness akin to that of old age, but from 

 which recovery is usually certain, on cessation of the excess of 

 work. The third may be of long duration with ultimate revival, or 

 it may lead to extinction. It may be compared with a serious ill- 

 ness, from which recovery is doubtful. 



1. Mutilation. When a river has its headwaters or branches 

 cut off by capture or diversion, it quickly begins to age. From a 

 youthful, cutting or degrading stream it changes to a mature one, 

 where graded conditions are maintained. By further reduction, old 

 age is reached, when the river begins to aggrade its valley, by drop- 

 ping what debris is brought to it. The river Bar, an affluent of the 

 Meuse, is such an old river, aged by mutilation, which resulted 

 in the loss of its headwaters. It now staggers through a debris- 

 clogged valley, which is out of proportion to its present size. 

 The Meuse itself may be considered at least a submature river, 

 just able to maintain its course, which in this case means cut- 

 ting through the rising ground, which is equivalent to carrying 

 away all the debris brought to it without cutting, in a stationary 

 region. 



2. Sudden overloading of a mature or submarine river may also 

 produce the phenomena of old age, by causing the river to aggrade 

 its floor. This may occur while the river valley is still a long way 

 from entering upon the state of old age. The gravel terraces of the 

 Allegheny, Monongahela and other Pennsylvania rivers, which rise 

 about 200 feet above the present stream channel, have been ex- 

 plained as remnants of high flood-plains due to aggrading of the 

 valleys by the overloaded Allegheny in early glacial time. The 

 debris was largely supplied to the river by the melting ice. Even 

 the tributary streams were forced to aggrade their valleys from the 

 mouth upward. Since then the rivers have cut down again through 

 this old flood plain, leaving the terraces on either side to mark its 

 former elevation. (Shaw, 35.) Similar examples are found in 

 New England rivers. 



3. Droivning of rivers also brings on the phenomena of old age. 

 This drowning may be due to overflow of a lake to which the river 

 is affluent, or to subsidence of the coast or to other causes. Ex- 

 amples of drowned coastal rivers are the Hudson, the lower St. 

 Lawrence, and numerous other rivers of the North Atlantic coast. 

 No erosion is accomplished by these rivers ; they have no apprecia- 

 ble current of their own, the movements of the water being regu- 



