IX, A. 3 Dalburg and Pratt: Iron Ores of Bulacan 211 



SEDIMENTARY EOCKS 



Post-Miocene formations. — Alluvium is developed along Angat 

 River in the vicinity of Norzagaray and Angat, between which 

 towns the river shifts across a wide valley floor. The town 

 of Matictic above Norzagaray occupies an alluvial flood plain. 

 The alluvium in this vicinity, owing to its position just at the 

 point where the swift mountain stream debouches upon the 

 plain and in consequence loses much of its transporting capacity, 

 is made up largely of coarse gravel with subordinate proportions 

 of sand and clay. The erosion of a comparatively large area 

 of complex geologic formation in the Cordillera has resulted in 

 a wide diversity of rock types in the gravel. 



The older gravels indicated in fig. 2 occur at an elevation from 

 25 to 35 meters above the present level of Angat River, and 

 attain a maximum thickness of about 10 meters. They are 

 not confined in their distribution to a former valley of this 

 stream, but are found uniformly north and south of Angat along 

 the eastern edge of the Central Plain. In Nueva Ecija Province 

 to the north, these gravels have been exploited in a small way 

 as placer-gold deposits, and in the vicinity of Angat itself fine 

 gold can be obtained from them by washing. In contrast with 

 their extended occurrence to north and south, the gravels do 

 not persist over more than a few kilometers laterally. Their 

 western boundary coincides roughly with that of the area 

 mapped, while to the east they do not continue beyond the barrio 

 of Sampaloc. 



The gravels are probably to be looked upon as overlapping 

 alluvial fans formed at the margin of the cordillera during 

 earlier stages of erosion. The individual fans have been more 

 or less commingled and spread out, probably through the lateral 

 shifting of the streams flowing across them. There is a notably 

 larger proportion of light-colored siliceous rocks in the older 

 gravels than in the modern alluvium, and it may be assumed 

 from this that at the time of their deposition erosion in the 

 mountains was confined in its action to a horizon represented 

 by the present ridges of altered and silicified effusives. No 

 attempt has been made to differentiate the relatively small areas 

 of alluvium and of older gravels from the underlying Pleistocene 

 formation on the geologic map. 



The Central Plain is built up of a series of tuffs, clays, sands, 

 and gravels, which attains a general thickness of at least 300 

 meters. This series of rocks occupies the western portion of 

 the area shown on the map, and overlaps on the Miocene beds 



