318 Transactions. 



Sculpture of the Sounds Block. 



The Tuamarina Valley differs from its neighbours only in having, at its 

 head, a low pass. In other respects it is a typical example of the valleys 

 which, before the subsidence, drained this mountainous district. 



The present Sounds area had then been, so far as one can now judge, 

 dissected to the early mature stage by normal agencies in a single erosion 

 cycle. There are many outrunning spurs both in the Tuamarina Valley 

 and in Queen Charlotte Sound which show very even and apparently 

 horizontal crest-lines suggesting successive cycles in the sculpture of the 

 valleys ; but it is significant that flat-topped remnants are absent. The 

 writer prefers to ascribe these even-crested spurs, and the apparent co- 

 incidence of level of neighbouring spurs which is sometimes observed, to 

 subequal spacing of parallel streams determined by systems of master 

 joints,* and to assume, in the absence of other evidence to the contrary, 

 that the Sounds valleys were the work of a single erosion cycle. 



The relief was strong (2,000 ft. to 4,000 ft.), and hill-slopes and valley- 

 sides were steep. The valley-floors, however, were graded and broadly 

 opened, flood-plains being developed nearly to their heads. The amount 

 of subsidence which ensued has not been sufficient to submerge the flood- 

 plains entirely. The town of Picton, for example, has been built upon 

 one unsubmerged remnant at the head of a tributary of Queen Charlotte 

 Sound, while another tributary with its flood-plain still unsubmerged opens 

 upon the Sound a little farther east as Waikawa Bay (see fig. 3). 



The subsidence which allowed the sea to enter this valley-system was 

 local. The extent of the block affected by it has not been investigated, 

 but it is important to note that it did not extend beyond the broad valley 

 of the Wairau, for southward and eastward there is abundant evidence of 

 recent uplift. Possibly movement took place along a fault-plane situated 

 somewhere in the Wairau Valley, or perhaps the explanation is to be found 

 in the formation of a flexure along the same line. 



Amount op Subsidence. 



The amount of subsidence in the Sounds block may be arrived at to 

 a rough approximation. The observer who enters one of the Sounds by 

 steamer may be tempted to produce the steep sides downward, in imagina- 

 tion, until they meet at the bottom of a V-shaped gorge thousands of feet 

 in depth — a veritable caiion. As mentioned above, however, flood-plains 

 exist at the heads of the valleys, indicating that they were graded, flat- 

 floored, and broadly opened. Into such valleys a moderate amount of sub- 

 sidence will allow the sea .to enter and penetrate far. A rough estimate, 

 based on probable grade, places the subsidence at between 250 ft. and 500 ft. 



It is possible, also, to arrive at a rough estimate from a consideration 

 of the present depth of water shown by soundings. The method can, 

 however, be applied only if it can be shown that the accumulation of marine 

 sediment in the valleys since subsidence is probably not great. 



The amount of deposit will be small if — (1) subsidence took place re- 

 cently ; (2) tidal currents have sufficient strength to keep the finer sediment 

 in suspension and carry it out to sea ; and (3) the area of deposition is large 

 compared with the area supplying sediment. 



* See Hobbs, W. H., " Repeating Patterns in the Relief and in the Structure of the 

 Land," Bull. Geol. Soc. Am., vol. 22, 1911, p. 123. 



