CONDITIONS OF POST-TERTIAKY EROSION 335 



is intended to represent the position of the coast at a somewhat later 

 period. 



POST-TERTIARY ELEVATION AND GORGE CUTTING 



The middle and late Tertiary time, as indicated above, was occupied 

 by a period of erosion, with the reduction of much of the region to the 

 condition of a peneplain. In the late Tertiary or Pleistocene the region 

 was again elevated, this time probably without deformation of its sur- 

 face, although there :fnay have been a slight arching of the isthmus on 

 the northwest-southeast axis, and possibly also an arching on a subordi- 

 nate axis west of the present lake basin. The total elevation was prob- 

 abl}^ between 200 and 300 feet. The immediate effect of this uplift was 

 to stimulate the streams to renewed activity. They began at once to 

 trench the peneplain and the broad baseleveled valleys which they had 

 formed in the preceding period. 



The consequences of the uplift were necessarily first felt in the lower 

 courses of the streams, and their valleys were there first lowered to the 

 newly established baselevel. Thence the deepened channels were cut 

 backward toward their headwaters. In the valley of the river which 

 occupied the present position of the San Juan from Castillo eastward, 

 various phases in the process of reduction were present. In the lower 

 course of the stream a broad valley was developed with only a few iso- 

 lated remnants of the former plain remaining. This extended upward 

 as far as Tamborgrande. From Tamborgrande to the Boca San Carlos 

 the valley was rather broad, but the adjacent hills retain distinct evi- 

 dences of the former peneplain, and wherever the rocks were unusually 

 hard the valley of the stream was correspondingly restricted. Between 

 the Boca San Carlos and the Continental divide, which was then near 

 the present jDOsition of Castillo, the stream was comparatively small and 

 flowed in a narrow gorge. Its channel was cut down to a rather low 

 gradient backward to the present position of the Machuca rapids. At 

 this point was the junction of three branches, probably of nearly equal 

 size, occupying the valleys of the Infiernito, the Machuca, and the pres- 

 ent San Juan. 



The tributaries of this river also cut down into the old valleys, and 

 the extent to which they succeeded in lowering their channels varied 

 with their position and size. Naturally those nearest the mouth of the 

 stream were earliest stimulated to renewed activity by the lowering of 

 the trunk stream into which they flowed, and hence these had the longest 

 time in which to effect the lowering of their own channels, while those 

 nearer the headwaters of the trunk stream were not materially affected 

 until late in the gradation period. Thus the tributaries of the San Juan 



