PORT NICHOLSON

Dominion, Volume 15, Issue 1, 26 September 1921, Page 9

PORT NICHOLSON

GENERAL TOPOGRAPHY RAISED EEACHES AND DROWNED VALLEYS On Saturday evening Dr. C. H. Cotton, D.Sc., F.N.Z., Inst., delivered a lecture in tho Trades Hall on Port Nicholson and the Hutt Valley, and screened a number of slides in illustration of 'his subject. The lecturer dealt with, the general topography of the district, and described Port Nicholson as being simply an area of subsidence of recent date (.geo. logically speaking). "When sinking took place,” said Dr. Cotton, "the ridges and valleys round about had already been carved by erosion into shapes very similar to those they' now have. Beneath the harbour bottom, which consists of mud, sand, etc., brought down chiefly by the Hutt River, there must be buried ridges and valleys. Most of the boundaries of tho harbour depression were formed by simple bending down (warping) of the hilly surface; but along the lino that became the north-west shore of the harbour and ia now followed by the Hutt Road, there was a shatterbelt, i.e., a zone of rock weakened by faulting that had taken place long oges previously. Along the line of small resistance thus formed the land broke instead of warping. A fault was formed. At the surface it shows as a fault-soarp, which must at first have been a continuous wall descending from the hill crests to the original bottom of the harbour. Now, however, the scarp is out into triangular sections, or facets, by ravines. A renewed movement on tho fault, later than that which initiated the harbour depression has caused all the smaller ravines along the ecarp to “hang”; that is to s»y, their mouths open at a considerable height about sea-level." The lecturer spoke of general evidence of uplift seen in raised beaches uplifted at the time of the 1855 earthquake. Ho referred to similar, but higher, beaches around Cape Turakirae and the west shore of Pnlliser Bay. At Baring Head there are two very distinct remnants of beaches nt heights of 80ft. and 160 ft. respectively above the Baring Head platform. Eastward of Baring Head, between tho Wainui-o-mato. and Orongorongo Rivers, the two beaches do not survive but above the eastward continuation of the Baring Head platform there are other well-presei’ved remnants at a much greater height. None of the platforms can be traced with certainty beyond the Orongorongo River and around Capo Turakirae, which forms the end of a narrow mountain ridge, but the profilo of the ridge is smooth towards the end, and there is a faint suggestion of it bench rounded and lowered to tho margin by erosion. Slides illustrating these r.igh shore platforms, indicating old shore lines both east and west of the harbour entrance, were shown, as were slides affording other evidence of intermittent uplift in the forms of hills and valleys. Dr. Cotton then discussed the various wavs in which, harbours were formed and stated that Wellington Harbour and its continuation, the Hutt Valley, hud been formed by local down warping, complicated bv in-break along a line of fracture which extended north-eastward up the Hutt Valley and south-westward to the sea. This had left a wall-like "faultscarp” from Thorndon to Petone. and up tho west side of the Hutt Valley. He referred to the evidence for the faultscarp and the line of weakness, marked bv peculiar valleys, which determined it. "Tim Upper Kaiwarra Valley and the upper vnllcv of Silver Creek." he said "arc quite straight and are inn line not onlv with each other, but also with the fault-scam forming the north-west shore of Port Nicholson. They arc presumably guided l>v the same zone of weak rock (shatter-belt). The present vallleys are entirely the work of stream erosion, however, tho streams that cut them haying first come, to occupy their lines by working headward along the slietterOther boundaries of the harbour were declared to be obviously not fault-scarps n.nd contrasted strongly with that describi’d They were evidently formed by tho tending down of the previously hilly surface so as to submerge the central portion. The evidence of this "drowning” was to te seen in the partial submergence of hills and valley in the Miramar Peninsula. Lvall Bay, Evans Bay, and Gotland’s Valley. The lecture was extremely interesting and was made more so by the slides, diagrams. and maps that were screened. Dr. Cotton was accorded a hearty vote of thanks.