SUBCUTANEOUS EROSION 



35 



are composed of alternate layers of limestone and marl. The latter 

 substance crumbles and flakes, the limestone rock cap is undermined 

 and breaks away fragment after fragment ; the upper portions of the 

 containing walls, thus exposed to air, frost, and rain, continue no longer 

 perfectly perpendicular they tend to become exceedingly acute slopes. 

 Shielded by soil only, the unexposed portion of the cliffs, where there 

 is no possibility of wear and tear, remains absolutely perpendicular. 



X 



> MII.'W ' <>M * l ^''^ /> %jii ^' 



Limestone rock cap undermined. 



At length our conjectural section of once even eastward-sloping range 

 is left an open gorge stripped of its original contents, exposing a slope 

 studded with fragments of undermined limestone. 



There have now been traced three stages of development in the 

 underground drainage system of Tutira : the first, erosion by percola- 

 tion of rain-water only, together with a deepening of the valley within 

 upright walls ; the second, erosion by percolation of rain-water plus 

 erosion by springs, together with a deepening of the valley within 



