NATURE NOTES.

New Zealand Herald, Volume LXVII, Issue 20508, 8 March 1930, Page 1 (Supplement)

NATURE NOTES.

CRUMPLED ROCKS.

BV J. DRUJIMOND. F.L.S., F.Z.S.

On a sea-cliff at Fitzroy Bay, Wellington Harbour, there is a small but excellent example of rock formation wellknown to geologists, sometimes on a vast scale. It illustrates the fact that rocks, hard and solid as they are, may, by sufficient side-pressure, be crumpled aud corrugated. Enormous side pressure on the rocks at Fitzroy Bay, originally horizontal, squeezed them so tightly that they crumpled like a piece of corrugated iron, even like a piece of thick cloth. The result is an archliko formation, about 22ft. high at top and about 40ft, wido at the bottom, from limb to limb of the arch. Geologically this is an anticline. The pressure that creates one of these massive ridges sometimes creates its opposite, an inverted arch, like a huge trough. This is a syncline. There is no inverted arch at Fitzroy Bay. In some places, arch and inverted arch, anticline and syncline, may be traced over a large area, running up and down in the shape of a sheet of corrugated iron, regularly ridged and trougiied from end to end. At first glance it seems impossible that blocks of rocks, so massive, hard, resistant and solid, can be handled by Nature in this cool nonchalant way, with as much ease as a person folds cloth in the hands; but here at least geologists rest their explanations on facts as solid as tho "rocks themselves, only more so. Most rocks composed of sediment carried by water were laid down horizontally Oil the floor of a sea or of a lake. These are sedimentary rocks. Their composition is sand, gravel, clay, and other materials torn from the land and transported bv -water. A mudbank, mixed with sand and gravel, subject to pressure, perhaps to heat, and completely cemented, becomes rock. The sedimentary rocks form ninetenths of tho exposed land area of the earth. They often are found' tilted from their originally horizontal position to every angle. Some even are vertical. More than this, some have actually been overturned. Almost any material bends without breaking under sufficient pressure for a very long time. This is so with rocks. Geologists, finding sedimentary rocks forming hills and lofty mountain-chains, are forced to the conclusion that those rocks were thrust up by a stupendous power. The only explanation of the tilting and the folding, is that the rocks were crumpled by sidepressure exerted by the earth, pressure so great and so long that nothing can» resist itl The time required to produce the crumpling may stagger the imagination, but it comes well within the time-measurements provided by geological knowledge. careful experiments have shown that _ the crumplings may be produced artificially in the more plastic materials in a short time. Nature has crumpled some rocks into folds completed in a piece of rock held in the hand. Other folds stretch from end to end or side to side of a whole continent. There are folds in the Appalachian Mountains in America, twenty miles wide and 100 miles long._ Other forces besides pressure operating, the ridges sometimes disappear, even become troughs. The troughs sometimes become hills. This is the result of erosion. As a ridge—an anticline, to keep in mind the geological term—is un elevation, water runs down it and makes a channel along the slope. The channel- is occupied "bv a stream, which, entrenching itself, cuts in deeper and deeper. In time, the ridge is all washed away, perhaps lower than the rock that once formed the trough, which, now standing out above the surrounding country, be- f comes a hill. In that position, it is' open to attack from erosion in its turn. It - may be reduced to the level of the surrounding country, or to its original state as a trough—that is, a valley. Time in this way brings many changes. There is ample evidence that they took place in the past. As the same old forces are pressing tho rocks, and the same old agents of erosion are at work, the same sort of changes are taking place before our eyes, but .so indescribably slowly that we cannot see them, or can see them onlv through the mind's eye, aided 1 by knowledge and sound reasoning. ' ' "■■■■. Crumpled and folded rocks sometimes contain fossils and pebbles deformed, extended, or sheared in tho direction of the crumpling or folding by the pressure on the rocks. Crumpled rocks have been compared to sheets of corrugated iron. Ridges and troughs in corrugated iron are symmetrical. Symmetrical crumpling is not usual in rocks. In the. normal type, one side of the fold :s shorter and steeper than the other. As a matter of fact, a complete arch, like the arch at Fitzroy Bay. is not often seen, except in the small examples it represents. This is accounted for by the fact that the tops are soon worn away. Nature has left the remains of an "immense trough—syncline, geologically—twenty miles wide in the KawhiaHuntly district in the North Island. It is complete south of ' Kawliia only. Northward, its western wing was depressed below sea-level by earth movements, which, ages ago, hewed the features pf New Zealand, leaving them very much as they are now. Some geologists believe that the Southern Alps were uplifted largely by folding, first by an intense folding of the rocks. This was followed by erosion, continued for a vast time, tli© old range being planed down by the erosion. The second uplift, which produced the modern range, probably was accompanied by onlv minor folding. The structure of the 'Southern Alp? was compared by early geologists to the structure of a huge anticline, whose western half was removed bv erosion or by subsidence, but Mr. P." G. Morgan, a former director of the New Zealand Geological Survey, said he believed that the supposed western side of the great anticline never was there. All the principal mountain chains in the world have resulted from the uprising of folds. Mountain-making. 1 one of Nature's biggest jobs, has been described by Professor J. Park, of Dunedin, as the result of local folding "of the rocks in regions wbero the uplift is faster than the effects of erosion. "If tho erosion was equal to the rate of uplift, the folds would not form highlands or features of bold relief.. Alpine chains are regarded as an expression of comparatively rapid folding. # The great height and the rugged contours of the Rocky Mountains, the Himalayas, the Andes, the Pyrenees, and the Swiss Alps, _ are evidence of youth, for even mountain ranges have thoir stages of childhood, youth, middle-age and old age. Recently and rapidly uplifted in geological* time, the erosion that-has attacked them has succeeded only in eroding the crests of their folds into narrow serrated ridges, deep valleys and profound gorges. _ According to Mr. Morgan, there is evidence that the Southern Alps, as we see them, were not a mountain-chain before the Miocene Period. This, was in the middle of the Cainozoic Era, and was merely some 19,000,000 years ago. In remotest geological times, when tho world was very young, there were, great alpine chains. * their only vestiges now, according to Professor Park, are worn down stumps. Many of these have lain buried beneath piles of sedimentary rocks fov countless aeons. Here and there the buried stumps have been exposed by recent erosion, or . disclosed by deep boring. Most of tho present mountainchains occupy the ruins of Palaezoie mountain-chains, which raised their proud heads in excellent majesty, perhaps 500,000,000 years ago.