212 7. Bowman — Physiography of the Central Andes. 



Confirmatory evidence of general uplift with respect to 

 which the present drainage has not yet been adjusted is found 

 in the abnormal profile of the thalweg of Quebrada Quisma an 

 eighth of a mile eastward of the edge of the plateau. From a 

 relatively flat upper portion a change is made to a steeper 

 gradient virtually on the edge of the plateau.* This condition 

 was accurately determined by field measurements. 



A second feature interpreted as a probable indication of two 

 periods of displacement of the blocked sections of the pene- 

 plain, is worthy of note here. It suggests that these displace- 

 ments were accompaniments of the general uplift that the 

 region experienced and therefore but the different expression 

 of a cause common to the two adjustments. It is the appear- 

 ance of the edge of the block illustrated by fig. 7. The upper 

 part of the scarp descends by a somewhat smooth and rela- 

 tively gentle slope to a shoulder of more or less definition 

 where the descent is continued with distinctly increased steep- 

 ness to the foot of the scarp. Tiie effect on the drainage lines 

 is to give them all reverse curve profiles but slightly read- 

 justed to the present outline of the edge of the block. This 

 appearance is very striking as one views the block of fig. 7 

 from the lower slopes of the twin peaks of Huasco. It also 

 comes out with great distinctness as one descends the trail 

 from the southwest to the spring at the western edge of the 

 basin and passes in review the various features indicated. 



It is concluded that this relation of upper and lower slopes 

 signifies two periods of faulting separated by a pronounced 

 interval of stability in the relative positions of the affected 

 blocks. The upper slopes would, if this view be correct, repre- 

 sent the graded condition which was reached after the first 

 period of faulting, while the renewal of the fault would again 

 define the face of the block by recreating the fault scarp. 



In a desert region of active faulting the opportunities for 

 the occurrence of antecedent drainage are obviously dimin- 

 ished as compared with the possibilities of a humid region. It 

 therefore did not seem probable when the field was first 

 examined that any clear cases would be discovered where 

 the present attitude of the block would be disregarded by a 

 transverse stream. Indeed, all of the first streams examined 

 about the Lake Huasco district showed strong conformity with 

 the dominating slopes. It was, therefore, very gratifying to 

 find, in the last inspection of the field on the return journey 



* An interesting topographic complexity is exhibited in the last three- 

 quarters of a mile within the margin of the plateau. It consists of a second 

 terrace distinctly below the level of the first and of decidedly limited devel- 

 opment. The origin of this second terrace is, however, not of immediate 

 interest here and will be described in a later paper. 



