i8 W. M. DAVIS 



with mature and old shoreline features; but here, as the action of 

 waves is essentially in a horizontal plane, degrade and aggrade 

 should be replaced by retrograde and prograde. The various river 

 terms may be modified by such adjectives as revived or rejuvenated 

 — Heim long ago used the word " Neubelebung " — to suggest 

 idea that a recent upheaval has brought about a youthful stage in 

 a current cycle after a more advanced stage in a previous cycle. 

 Phihppson introduced the important idea that revival of stream 

 erosion in the upper part of a river system may be brought about 

 as well by a down-flexing or down-faulting of the lower part of the 

 system as by a general upHft, for thereby the headwaters will be 

 enabled to incise their former valley floors. 



Phihppson also developed the idea of migrating divides, and the 

 resulting rearrangement of water courses, as in the case of a higher- 

 level stream that is tapped by the headward growth of a lower- 

 level stream at what I have called the "elbow of capture" and thus 

 divided into two parts, the upper part being diverted and the lower 

 part beheaded. As the lower-level stream head approaches the 

 elbow of capture, the divide between them creeps toward the 

 higher-level stream; at the time of capture the divide leaps 

 around the basin of the diverted stream; then as a rule, again 

 creeping slowly, it still further beheads the beheaded stream: a 

 short stream is thus developed between the retreating head of the 

 beheaded stream and the neighborhood of the elbow of capture; as it 

 flows in a back-handed direction it may be called an inverted stream. 



Captures of this kind are largely the work of subsequent streams : 

 they may be described as eventual, imminent, recent, and long-past 

 or remote. As they are accomplished, a drainage system departs 

 more and more from its initial consequent pattern and becomes 

 more and more fitted or adjusted to belts of weak structure, and 

 at the same time the surviving ridges and divides become adjusted 

 to belts of resistant structure. The reduced volume of a beheaded 

 stream cannot develop meanders of the same size as those which it 

 followed, with larger volume, before its beheading and by which its 

 valley may have been given an incised meandering pattern; hence, 

 as the reduced meanders are too small to fit their valley curves, they 

 may be called underfit. Conversely, the capturing stream, being 



