Cotton. — The Tuamarina Y alley. 321 



by a fault which separated a heaved Sounds block from a thrown block 

 which underlies the Wairau Plain and the extension of Cloudy Bay which 

 formerly occupied its site. It will be noted that the fault which it seems 

 necessary to postulate here, while following approximately the line of the 

 hypothetical fault or flexure along which the Sounds block has recently 

 subsided, had its downthrow in the opposite direction. It must also be 

 supposed to be of earlier date, antedating the dissection of the Sounds block. 

 The bold scarp, part of which is shown in fig. 4, and the line of which 

 is indicated in fig. 3 as " Old Shore," cannot be regarded as preserving any 

 remnants of the original fault-scarp. The line of cliff-facets might be 

 produced — (I) by recent faulting, (2) by lateral cutting by the Wairau 

 Eiver, or (3) by marine erosion. The continuity of its line with the actual 

 sea-coast a few miles to the north-east indicates with a fair degree of cer- 

 tainty that it is marine erosion which has been responsible for the cutting 

 of the actual facets. The rock platforms of the old shore are buried beneath 

 alluvium. 



Changes brought about by the Advance of the Wairau Plain intc 



Cloudy Bay. 



The Wairau Plain has been built forward along the old shore either 

 as an ordinary delta or, as suggested by Mr. L. J. Wild,* filling a great 

 lagoon, a portion of Cloudy Bay enclosed by a spit of gravel supplied by 

 the Awatere River. 



The advancing alluvial deposits closed the mouths of the Tuamarina 

 and neighbouring valleys, and at Tuamarina the surface of the plain is now 

 about 20 ft. above sea-level. 



Deposits in the Tuamarina Valley. 



The Tuamarina Valley was thus converted from an arm of the sea into 

 a lake or lagoon. Into the lake the main stream and its tributaries would 

 continue to pour their loads of waste. There would be now, however,, 

 no tide to aid in the distribution of the waste and to carry the finer material 

 to sea. Before the mouth of each small stream, therefore, a delta of the 

 coarser waste would be built, while the finer would be spread over the floor 

 of the lake. To the local supply would be added during floods a generous 

 contribution from the line silts of the Wairau as the Wairau Plain was built 

 up from sea-level to its present height of 20 ft. The result would be that 

 the lake would be filled, forming a flat at sea-level, above which would rise 

 the sloping fan-like surfaces of the deltas of the main and tributary streams. 

 After that stage was reached the waste brought in would all be dropped 

 as the streams emerged upon the flat, fans would be built forward, and 

 farther up each stream aggradation would take place, the fluviatile beds 

 all being laid down parallel to the adjusted grade of each stream, and gradu- 

 ally extending outward over the sea-level flats. 



At the valley-mouth the surface would slope gently up to the level of the 

 Wairau Plain. 



If the above reasoning is correct, the deposits in the valley should have 

 the structure indicated in fig. 5, from which, for the sake of clearness, the 

 delta and fluviatile deposits of side streams are omitted. They should 

 have a similar structure to that of the deposits of the main stream. 



* Oral communication. 



11— Trans. 



