PHYSIOGRAPHY OF THE PHILIPPINE ISLANDS: H. 17 



by spurs running out from these ridges. The streams here have gentle 

 gradients and gently sloping banks. In the central part of this valley 

 outcrops of andesite are found, one of which shows a wide zone of 

 pyritized attrition clay, with a northwesterly strike, the result of faulting. 



The two streams draining this valley flow to the southward, cutting 

 through the Matarem range which here runs east to west, in steep 

 canons, uniting at the barrio of Itbod. The eastern of these two 

 streams can be followed from Itbod to within a short distance of the 

 point where the broad valley begins. It flows through a very narrow 

 gorge generally less than fifteen meters in width, with walls over sixty 

 meters in height. "Waterfalls are frequent. 



There is no reason why the valley should not be readily explainable 

 by superimposed drainage. The presence of a fault the strike of which is 

 parallel to the long axis of the valley is probably sufBeient to account for 

 this. A shattered fault zone would readily yield to erosion and be worn 

 down to the grade determined by the rate of cutting of the stream through 

 the well cemented material to the south. Hence, the part of the stream 

 and its tributaries in the region of the fault zone would be practically 

 at grade, only cutting down the valleys as the deepening of the caiion 

 gave them new power. Thus, there is a gradual "sinking down" and 

 preservation of mature topography from a previous cycle in a present 

 extremely 3'outhful stage. It seems strange that the stream draining 

 the valley should not flow along the line of the faulting to the southeast, 

 where the hills are lower, instead of directly through the highest part of 

 the ridge. As it does not, there is evidence that the drainage conditions 

 which existed before the uplift began, are now superimposed upon the 

 younger topograph}^ Eventually, if conditions remain unchanged a 

 stream working up along the fault zone from the southeast will capture 

 the headwaters of the present streams. It may already have done so in 

 part, but without a topographic map the dense vegetation makes it im- 

 possible to secure any grasp of the details of the physiograjshy. Tlie two 

 streams which at present drain the valley are also engaged in a struggle 

 for supremacy. The eastern of these, having the shorter course, can 

 make steeper grades and hence will eventually have the advantage, but 

 at present this seems partly neutralized by the otherwise more favorable 

 situation of the western one, which receives the drainage from the Mata- 

 rem Eange. However, it seems probable that the eastern stream has 

 already captured an eastern l)ranch from its neighbor. 



A peculiar feature of drainage conditions in general is the small 

 amount of woilc accomplished by the streams in a region of sucli heavy 

 rainfall. The older topograjihy of Sabtan is of pre-Miocene age and is 

 at an elevation of over 300 meters, hence it seems strange that any of 

 it, far less such a large belt, should still exist. One reason for this 

 condition is undoubtedly the smaller drainage basins of these islands as 



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