23 > 5 Dickerson: Baguio Plateau 435 



nidad Water Gap is very evident and this elevation of 1,450 

 meters (4,850 feet) fits in nicely with the low-lying hills of 

 Baguio beyond (Plate 11, figs. 1 to 3). When the stream cut 

 this terrace at 1,450 meters (4,850 feet) the country evidently 

 was at an elevation near sea level, and the course which the 

 stream picked out across a country that was then largely covered 

 by the tuffs and tuff-breccia of the Baguio formation and its 

 alluvial derivatives was an accidental one. After uplift, the 

 stream had sufficient volume to maintain its course and within 

 its former wide valley now represented by the residual at 1,450 

 meters (4,850 feet) dug a canon for itself. With uplift, the 

 erosive power of the stream was greatly increased and this 

 canon was further developed. This downward cut of the stream 

 may be likened to a band saw against which a board is thrust. 

 As the saw descends through the board, a knot, soft dry-rotted 

 spots, and wood of medium grain are successively encountered. 

 A sharp narrow slot is cut into the knot, the saw may be bent- 

 from its course as it passes through the soft dry-rotted portion, 

 while in the clear-grained wood a slot of moderate dimension is 

 cut. Similar conditions, with differences in hardness, occur in 

 the vicinity of Trinidad Valley. The andesitic tuff -breccia north 

 of Trinidad corresponds to the knot. The compact, resistant 

 coralline limestones of the water gap at the south entrance of 

 Trinidad Valley represent another such development. The soft 

 incoherent tuffs on the west side of Trinidad Valley are analo- 

 gous to the soft dry rot in the timber. The unusually hard 

 andesitic tuff-breccia north of Trinidad acted as a temporary 

 dam, that is, in technical language, set up a local base level of 

 erosion. The stream above this dam was at times temporarily 

 restrained from downward cutting and then began to swing 

 from side to side, developing broad meanders, now represented 

 by somewhat distinct terraces on the side of Trinidad Valley. 

 Since the softer material was on the western side and the harder 

 material on the eastern, this horizontal cutting was chiefly con- 

 fined to the western side. 



Baguio Plateau was not elevated at one stage, since two 

 terraces noticeable on the hills bordering Trinidad Valley rep- 

 resent in all probability two uplift stages. The third uplift is 

 probably represented by the present valley development. As 

 Trinidad River cuts through the north water gap again the dam 

 will be lowered and in future geologic time Trinidad Valley will 

 disappear. This explanation is chiefly physiographic, so direct 

 geologic evidence of Pleistocene stream terraces containing 



