PROPOSED RODING TUNNEL

Nelson Evening Mail, Volume LXXI, 26 April 1937, Page 9

PROPOSED RODING TUNNEL

♦ j C [To The Editor] I | h Mr Pickup I am sure knows ten j times more about the local geology fojjj this district than I do, and as a practical j miner his opinion should carry much i weight with regard to the driving of j the 9700 feet long tunnel from the f Roding, The rocks may be all he claims, but nobody can be sure until the vvork is put in hand. My experience is that no estimates ' are so hard to make, and likely consequently to be wrong, as those connected with underground driving. There ( are so many uncertainties, and the roof , seepage that almost invariably occurs, ( will not only rust the pipes, but in- ( loosen and disintegrate the , roof and sides of the tunnel. Hence it | is just as well to face the possibility of having to spend another £IO,OOO or J £15,000 in protection works, like ( lining the roof and concreting the pipes. . Mr Pickup’s views about the absence of gullies, and that conical rounded , shaped hills, indicate hardness and not softness in the rocks composing them, . interest me very much, as I have always thought exactly the reverse. It is possible that we misunderstand .one another as to what would be classified as a hard rock. In New Zealand as a whole the rocks are soft like they are in Switzerland, due to their recent formation. Consequently the two countries are not unlike physically. Both have high mountains and hills full of gulies, in both the denudation is terrific, though less severe in Switzerland owing to the more rapid growth and hardiness of the pines; both have been subject to recent glacial action, giving rise to lakes that radiate from the highest mountains, and have steep gorgy valleys leading to plains and deltas in [ake or estuary, such as we have round Tasman Bay, that make for unstable conditions in the river channels. 1 have been used to the older rocks of Cornwall and Devon, to those of Wales, Scotland, and the lake country of the north of England, i.e.. to the hard slates and conglomerates of the Silurian and Cambrian formation, and to the mighty schists, limestones, sandstones, and igneous rocks that abound everywhere. I don’t call the Maitai quarry rock, that shatters under a light hammer blow, a hard rock, and no architect would recommend it for building purposes I should imagine. Hence we had

;o build our cathedral with marble (1 1 should call it hard limestone), and i :arry it 100 miles. I come across a few hard outcrops in river channels and hilltops, but comparatively few. and this seems to be characteristic of most of New Zealand over which I have travelled. Hence j the comparative absence of high cliffs.' 1 the tameenss of the sea shores, and many other features that those born in other lands miss in New Zealand. Even the hard basalts that I have seen in Tasmania, and that abound over much of Australia, are absent here. In my opinion the absence of gullies* on a hillside is more an indication of uniformity of texture and absorption, than of actual hardness. Thus a soft uniform rock like chalk, that absorbs the rain, forms open downs, free of gullies, and vertical cliffs on the coast line that actually harden with exposure, just as we see round this district gravel formations along the river channels standing verticaly as cliffs 100 ! feet high. It is the same in the mountain lime-1 stone districts of Derbyshire, the Mendips, the Downs at Clifton, and the Takaka Hills but in these cases chemical action, due to carbonic acid in the rain water, working on the less uniform texture of the rocks, comes into play, and the rate of disintegration ; varies more or less directly according j U the quantity of water passing over ; or through it, accounting for the caves, I gaps, springs, and gorges so character-1 istic of limestone country. The birth of a gully is piobably a! rotten vein of rock that weathers out 1 more rapidly than other parts around ; it, and so becomes a gutter. I should j like to hear what so keen an observer j as Mr Pattie has to say on so interest- j ing a subject. Perhaps the Roding so-callecl slates | are harder than 1 imagine. They will dv.ubtless stand, but will they disintegrate under the severe conditions tc which they will be subject —I am, etc., H. G. BOSTER BARHAM. Nelson, 23rd April.