206 



SCIENCE 



[N. S. Vol. XXXVIII. No. 971 



AKTIGRAVITATI05"A1 GRADATION 



NowHEEE on the face of our globe, we now 

 know, are the effects of the gradational proc- 

 esses so completely or so conspicuously exem- 

 plified as on the broad intermont valleys of 

 arid regions. There the graded plain is the 

 dominant feature of landscape. It attains a 

 degree of perfection that is wholly unknown 

 elsewhere. It is more even than is theoretic- 

 ally demanded of the ideal or finished pene- 

 plain. It is, as Passarge astutely remarks, 

 smoother than any peneplain possibly can be. 

 Yet never has relief element been so generally 

 misunderstood or so entirely overlooked. 



In the course of the wide discussion which 

 the subject recently has aroused in almost 

 every land it is fortunate that so many local- 

 ized illustrations have been so carefully 

 described. For the first time we are now able 

 to cite definite references. The present aspect 

 of the theme centers around the topic of local 

 dissection and terracing of the steeper slopes 

 immediately encircling many desert mountain 

 ranges — the belt designated by physiographers 

 as the hajada, the title being an adapted 

 Spanish name. 



The remarkable phenomenon of bajada- 

 terraeing does not appear, as urged by Salis- 

 bury, to be a necessary consequence of the 

 general lowering of the highland by stream- 

 action while the intermont lowlands are being 

 filled up, because some of the best examples of 

 terracing border broad plains having rock- 

 floors. For the same reason it does not appear 

 possible that there ever occurs during so-called 

 topographic maturity an adjustment by water- 

 action between one bolson and another adja- 

 cent but lower one which results in the terrac- 

 ing of the higher, as suggested by Davis. 

 There is little or no evidence to show that 

 bajadas were all formed during periods of 

 glaciation, as advocated by Barrell, since 

 some of the most typical forms of this class 

 are found surrounding low knolls near sea- 

 level and far below all possible altitudes of 

 glacial action in the region. Neither does it 

 seem likely that bajadas were constructed 



during interglacial epochs of materials which 

 accumulated in the mountains when the 

 latter were covered by ice, as argued by Hunt- 

 ington, for this does not explain the many 

 bajada-belts with rock-floors. Nor is it any 

 better to postulate a recent increase of tem- 

 perature and a difl^erent distribution and 

 amount of rainfall abetted by the advance- 

 ment of the area in the geographic cycle, as 

 proposed by Visher, for the terracing is now 

 going on before our very eyes at an astonish- 

 ingly rapid rate, and as quickly is it also com- 

 pletely obliterated. 



Terracing of desert tracts appears to be 

 confined mainly to the foots of the loftier 

 ranges; and its accomplishment is fully 

 described elsewhere. Under the ordinary con- 

 ditions of deflative action we would expect the 

 locus of maximum lowering to take place in 

 the middle part of the bolsons. According to 

 this recognition of conditions eolic erosion 

 necessarily operates from the lower to a higher 

 elevation. As shown by Professor Davis, the 

 winds in their action are not dependent like 

 water on the gradient of the land surface for 

 their gravitational acceleration; they may 

 blow violently and work efl^ectively on a per- 

 fectly level surface. Unlike water they may 

 also erode vigorously up-hill; and this is 

 exactly what they manifestly and constantly 

 do on the bolson-plains. 



Notwithstanding the fact that wind erosion 

 operates both up and down the slope there is, 

 owing to the peculiar configuration of each 

 basin-shaped tract, a preponderance of effect 

 on the up-slope part of the course. There also 

 appears to be a limit to the gradient on which 

 the wind is able to blow sands erodingly and 

 extensively up-hill, and this limit seems to lie 

 chiefly between a two and a four per cent, 

 gradient. It is for this reason seemingly that 

 the intermont plains are so smooth, so uni- 

 form in grade, so high in gradient. Eolic 

 gradation thus mainly works from a lower to 

 a higher level. The direction of greatest 

 activity is directly opposite that of stream- 

 work. It is mainly up-hill. 



Charles Keyes 



