POST-CRETACEOUS HISTORY OF WESTERN WYOMING 309 



other flats and terraces which are normally developed by the 

 stripping of weak strata from harder beds in horizontal position — 

 a process which is peculiarly effective in dry wind-swept plains. 

 This distinction must be made particularly in the broad basins such 

 as the Wind River and Green River basins, where the Tertiary 

 strata have been but little disturbed. It is necessary also to bear 

 in mind that the graded surfaces, which constitute the immediate 

 goal of all streams, are not level, either transverse to the main 

 stream or parallel to it. Therefore remnants of the same graded 

 surface generally do not have the same elevation in different parts 

 of the district, even though there has been no subsequent warping; 

 but such remnants should bear approximately the same general 

 relation to each other as corresponding parts of the modern flood- 

 plains bear to each other. 



The studies of Westgate and Branson, already noticed, show 

 that in the Lander district the topographic features reveal a definite 

 sequence of cycles which may eventually be extended to a much 

 larger region. The existing topography suggests a declining series 

 of phases, in which the earliest cycles proceeded to old age, the 

 intermediate ones to maturity, and the later ones only to youth. 

 This may well be more apparent than real, however, for if any of 

 the early stages had gone no farther than youth, the evidence of 

 their existence would have been destroyed in those later cycles 

 which went farther toward completion. As I am still somewhat 

 doubtful about the correlation of the features I have seen with those 

 described by Westgate and Branson, I present in Table II an inde- 

 pendent scheme of glacial stages and erosion cycles, not as a final 

 list, but necessarily as a provisional one. 



Below the elevated peneplain, which has already been described 

 as probably of Pliocene age, I find evidence suggesting at least 

 four cycles of erosion, one of which appears to be older than the 

 oldest glacial drift, while the rest are more or less associated 

 with glacial phenomena. The latest cycle, which is probably 

 complex in detail, is represented by the inner valleys with existing 

 floodplains and their immediate terraces. It was closely asso- 

 ciated with the last glacial stage. The phenomena of the various 

 cycles may now be discussed more fully. 



