Cotton. — Notes on W ellingtcm Physiography. 



297 



high facets, all in line, which strongly suggest a fault-scarp, but may be a 

 fault-line scarp. It is, of course, true that a vigorous stream, in the process 

 of excavation of a broad-floored mature valley, might itself cut back both 

 its valley-sides to this form, but the absence of bluffs on the south-east 

 side of this portion of the Hutt Valley negatives this explanation. 



This portion of the valley may perhaps be a Gfraben, bounded, as the 

 northern end of the Port Nicholson depression is perhaps also bounded, by 

 a fault on one side and a flexure on the remaining sides. Fig. 2 shows two 

 trough subsidences formed in this way along a single fault, and bearing to 

 each other a relation similar to that between the lower and upper plains 

 of the Hutt Valley. The se.a may be supposed to have access to the nearer 

 of the two troughs, and to fill it up to the. broken line. The broken line 

 in the farther trough represents the level of a lake occupying the trough, 

 and overflowing at the lowest point (the south-west end) along the fault- 

 scarp. If a river were already in existence flowing south-westward along 

 the fault-line, and if the subsidence took place rather slowly, the farther 

 trough might be filled with waste as rapidly as it was formed, a graded 



Fig. 2. — Diagram of Two Trough Subsidences along a Single Fault. 



gorge being at the same time cut along the fault-line between the two 

 troughs. When later a delta had been built out into the nearer trough 

 the system would bear a good deal of resemblance to the Hutt Valley. 



In the actual case the pre-faulting surface was of strong relief, and 

 unless the Hutt River already existed it is difficult, if not impossible, to 

 account for the present large drainage-basin of the river, for the amount 

 of the fault-movement seems insufficient to effect profound changes in 

 drainage. 



Thus we have several reasons for believing that before the Wellington 

 fault movement took place the Hutt River existed as a fault-line river 

 guided by the same old fault as the Upper Kaiwarra. 



Another indication of the previous existence of the Hutt is found in the 

 presence of a broad terrace along the north-west side of the river near the 

 mouth, evidently a remnant of the flood-plain of the Tongue Point cycle, 

 the front of which is formed by the fault-scarp. 



The continuous scarp that bounds the Hutt Valley on the north-west 

 side seems to be — at least, in part — -a fault-scarp. If it is true, however, 

 that the recent break occurred along the floor of an older valley, the fault- 



