The Nature Column.

Digger (Invercargill RSA), Issue 20, 30 July 1920, Page 6

The Nature Column.

("Stndent" will be pleased to receive notes on any branch of Natural History. Observations on birds, insects, plants, etc., will be eqnally weleome. If using a pen-name, will correspondents please enclose real name and address. ) A correspondent in the daily press remarks that our mountains are only good to look at and no use or comfort to any one. Surely for a country cousin he must be very unobservant, and his knowledge of Southland's climate must be small. As I pointed out last week the mountains on our western coast are the cause of a high rate of rain fall, and the air flowing over this province is t-hus drier than it would otherwise be. Yery few people in the country growl about lack of rain. As a general rule we complain of receiving too much: Then again our mountains are mostly covered with vegetation, which is in some places to be classed as luxuriant. When this vegetation is tussock, it affords good feed for hardy sheep. Many thousands of pounds comes from Southland's high country. Our friend has little time for grandeur and beauty, but even so the mountains have an L.S.D. value which any farmer should e able to' understand. Owing to the high rainfall the forest growth is great, and our farmer friend may come to bless the fact that the inaccesibility of the high country caused a large area of forest to be preserved, which migbt have been wam tonly destroyed. The mountain foresta act as huge reservoirs storing up water and providing an even supply to the rivers. Mountains have a large effect on rainfall, and naturally many rivers have their sources in them. The work performed by rain and rivers is of the highest irnportance. The rivers of Southland built up the rich river flats with material brought from the mountains. The rivers laid down the substratum of gravel upon which the rich silts were deposited in times of flood. Many of the best river flats owing to the fine drainage provided by the gravel do not require lime, this material being most often r.equired by the day terrace lands. In journeying from Invercargill up to the mountains it is possible to see illustrations of life history of a river. Near the mountains it will be observed that watercourses are narrow, and the sides steep and high. In many places the sides are vertieah This is the young stage of a river. Owing to its steep angle of descent the water with the aid of bits of rock files out a deep furrow of no great, width, and the course is fairly straight. Lower down the strearn swings from side to side, gradually causing the valleys to assume a Y shape, this is middle age, and as they get near ihe eoast the river winds about, has low banks, and the valley flattens out to something like a plain. This a mature valley. It is doubtful if a river could actually cause a plain as some geologists maintain, fo' it seems almost certain that a time would come when the slope was so slight that the growth of vegetation would stop tl.c movement of the soil down the slopes, Thi slope of the river itself would ako each a point at which the rate of flow would be so small that erosion would practically cease. The mountains are mostly formed by pressure, but hills and valleys are largely the work of rivers. The mountain summits of Central Otago are supposed to be all that is left of a great elevated plain, the present valleys thousands of feet in depth having been gouged out by the rivers. The rivers get their material for cutting mainly from the mountains. The intense ! cold in the winter causes the rocks to : contract, and at the same time expands as ice the water imprisoned in cracks. With the return of summer the rocks are subjected to expansion and contraction, as night follows day the work commeneed in the winter is carried on, the rocks being gradually shattered into fragments. These fragments roll down the slopes into the little valleys, and thenee they are corrie! down to the main stream. They are rolled and knocked about and gradually redueed to gravel and mud. This fine mud is so much new earth, and is carried to the sea or deposited on the flats during floods. The rain is responsible for a large amount of the breaking down process. Charged with carbonic acid from the air it continually beats on and attacks rock sur. faces dissolving the hard material carrying some parts in solution and washing the Tet i down the slopes. Low country is being continually enriched with these washing The land surface is always on the move towards lower levels, and vast quantities c" material are in motion. Some geologists hold that the solid rock of the mountains is slowly flowing outwards over the land.