Bell. — Fhij'^iofira iilni of Wellington Umbo 



yir . 



539 



natural angle of rest for the products of decay of such shattered rocks as 

 the argillites and grauwackes of Wellington. This steepness in itself, in 

 the absence of any other explanation, such as glacial erosion, marine denu- 

 dation, &c., is a physiographic proof of faulting. 



A second proof is found in the presence of clay terraces, elevated about 

 300 ft. above the present level of the sea, on the steep slopes of the Karori 

 Hills at the back of Tinakori Eoad. These terraces mark the silt-accumu- 

 lations at the place where the subsidence of the harbourward block forming- 

 part of the original complex graben was temporarily arrested. 



A still further evidence of the faulting is found in the fact that the 

 small watercourses, such as several of the small streams between Ngahau- 

 ranga Creek and Petone, do not enter the harbour at grade, as would be 

 the case with normal drainage, but instead flow over the lower part of the 

 fault-scarp in abrupt waterfalls. 



Perhaps the most graphic evidence of faulting, however, is found in 

 the gorged nature of all the streams along the western side of the harbour, 

 and in the practical absence of even narrow flood-plains on the largest 

 streams, such as the Ngahauranga. The Ngahauranga flows into the sea 

 over a small gravel fan which it has built out into the harbour. Above this 



Sea-level 



Section from Cape Terawhiti to Mount Papatahi, 



Positions of Fault-lines. 



INDICATING PrOBABLK 



flat the stream flows in a narrow V-shaped gorge-like valley with numerous 

 waterfalls. At its headwaters, where it reaches the level of the peneplain 

 or crest of the block dipping towards Porirua. it flows somewhat more gently. 



The general character of the Ngahauranga is in marked contradistinction 

 to drainage in a hilly or mountainous country unafiected by faulting. There 

 one would expect, with mature topography, a broader valley, the presence 

 of flood-plains, especially towards the mouth, and the absence of water- 

 falls, save at the headwaters. Of this nature, in fact, is the Porirua Stream, 

 which descends towards the inlet of the same name down the inclination 

 of the tilted block of the old peneplain to the west of the harbour. The 

 stream flows throughout most of its course at grade, flood -plains are 

 common, and waterfalls are infrequent, save at its headwaters. The 

 reason of the difference in character of the two streams will be understood 

 when it is remembered that the Porirua Stream flows down the natural 

 inclination of the block ; whereas the other flows contrary to this inclina- 

 tion, across and down the steep fault-scarp. 



The actual extent of the throw of the fault at the west side of the harbour 

 is probably not less than 5<)0 ft. 



The contrast between Porirua Inlet and Wellington Harl^our is as great 

 as that of the two streams which enter them. The inlet is mainly the result 

 of the depression of the lower part of a well-sculptured river valley, while 

 the harbour is primarily an area of complex tectonic subsidence, influenced 

 to some extent by possible fiuviatile erosion in the past, and by marine 

 denudation and fiuviatile sedimentation. 



