112 Gregory — Progress in Interpretation of Land Forms. 



ley (1865), Maw (1866), Wynne (1867), Whitaker (1867), 

 Macintosh (1869), Green (1882), Richthofen (1882), but 

 all of them were looked upon as products of marine work, 

 and writers of more recent date in England seem reluc- 

 tant to give a subordinate place to the erosive power of 

 waves. Americans, on the other hand, have been think- 

 ing in terms of rivers, and the great contribution of the 

 American school is not that peneplains exist, but that 

 they are the result of normal subaerial erosion. More 

 precise field methods during the past decade have 

 revealed the fact that no one agent is responsible for the 

 land forms classed as peneplains ; that not only rivers 

 and ocean, but ice, wind, structure, and topographic 

 position must be taken into account. 



The recognition of rivers as valley-makers and of the 

 final result of stream work necessarily preceded an 

 analysis of the process of subaerial erosion. The first 

 and last terms were known, the intermediate terms and 

 the sequence remained to be established. A significant 

 contribution to this problem was made by Jukes (1862 ). 22 



"... I believe that the lateral valleys are those which were 

 first formed by the drainage running directly from the crests of 

 the chains, the longitudinal ones being subsequently elaborated 

 along the strike of the softer or more erodable beds exposed on 

 the flanks of those chains." 



Powell's discussion of antecedent and consequent 

 drainage (1875) and Gilbert's chapter on land sculpture 

 in the Henry Mountain report (1880) are classics, and 

 McGee's contribution 28 contains significant, suggestions, 

 but the master papers are by Davis, 29 who introduces an 

 analysis of land forms based on structure and age by the 

 statement : 



"Being fully persuaded of the gradual and systematic evolu- 

 tion of topographical forms it is now desired ... to seek the 

 causes of the location of streams in their present courses ; to go 

 back if possible to the early date when central Pennsylvania was 

 first raised from the sea, and trace the development of the several 

 river systems then implanted upon it from their ancient begin- 

 ning to the present time." 



That such a task could have been undertaken a quarter 

 of a century ago and to-day considered a part of every- 

 day field work shows how completely the lost ground of a 



