Life in Lakes and Rivers. T. Macan T.

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СКАЧАТЬ glacier, and some glacial lakes are dammed up by a moraine which makes them deeper than if they were contained in the actual excavation alone. Glacial lakes, like rift valley lakes, are usually long and narrow and relatively deep; Windermere, for example, is 10¹/₂ miles long but only half a mile wide on the average, and 219 feet deep at the deepest point. Their sides tend to be parallel and any major irregularity in the shore line is often of more recent age. Lakes of this type were formed only in hard rocks where the relief was rugged, and steep valleys concentrated the glacier and directed its excavating effort to a circumscribed area.

      Also of glacial origin are the smaller lochans, tarns, and the small lakes in cirques, corries, or cwms which are often to be found near the tops of mountains. They are frequently circular in outline and they mark the place where the snow or ice piled up and a glacier took its origin.

      An ice sheet covering a plain did not excavate because its effort was dispersed and not concentrated, but it did give rise to lakes none the less. As might be expected these are of a different type; Loch Leven is an example and no fisherman requires a biologist or geologist to tell him that there is something fundamentally different between Loch Leven and the Highland lochs. As the ice sheet which covered Scotland began to recede, a large lobe of the glacier flowing down the Forth Valley became isolated in the centre of the Kinross plain. It was surrounded by clay, stones, boulders, and suchlike products of ice erosion washed along in the water from the melting glaciers, much of it coming through a pass in the Ochils from the Tay Valley. A considerable depth of this material was deposited on the plain, but in the middle there was this big block of ice melting slowly because of its large size, like an iceberg in the North Atlantic. When it finally disappeared it left a hollow where it had been sitting and this filled up with water to become Loch Leven. The shape of Loch Leven is quite different from that of either glacier-cut or rift valley lakes: it is not much longer than it is broad, one axis being 3²/₃ miles (5.7 km.) and the other 2²/₃ (4.1 km); its mean depth is only 15 feet (4.5 m.) and its greatest depth only 83 feet (25 m.).

      The Cheshire Meres were formed in a similar way to Loch Leven, although subsidence of the land surface also played a part. Outside Britain there are many lakes of the same type: two groups, which are referred to in later chapters because they have been studied in much detail, are the numerous lakes in the Wisconsin area of North America and the Baltic lakes of Denmark, Germany, Poland and U.S.S.R.

      Also characteristic of mountainous areas are the much smaller peat pools. These may occupy holes where stone for a wall or a house has been quarried or sometimes a rock basin of natural origin, but most of them are formed by the growth and then the erosion of peat. Some of the largest are to be seen on the Pennines, for the Pennines have flatter tops than the mountains in Scotland, Wales, or elsewhere in Britain, and it is on flat places that these pools develop. Vegetation, of which bog-moss (Sphagnum) is usually an important constituent, dies and accumulates over a long period of years, building up a bed of peat. At a certain stage, for reasons which are not at present understood, the peat becomes unstable, and hollows are eroded by the action of wind and rain. The surface becomes dotted with small pools and, as further erosion takes place, these coalesce. Finally a channel becomes eroded through the rim of the peat bed and all the water runs away. The building-up process then starts again.

      Larger bodies of water on the Pennines are few, apart from man-made reservoirs which are now characteristic features of the landscape.

      Lough Neagh in Northern Ireland is the largest sheet of fresh water in the British Isles, with a surface area of 153 square miles (393 km.²). It was formed in a way different from that of any of the other lakes so far encountered, and is volcanic in origin. There was no volcanic mountain like Etna or Fujiyama, but basaltic lava welled up from fissures in the ground. It flowed freely over the countryside and eventually solidified as a flat plate-like capping. Later it sagged in the middle and the depression so formed is Lough Neagh. The greatest depth is only 56 feet (17 m.), so it is even shallower than Loch Leven.

      There remain to be explored the more recent geological formations of south-east England, and on them there are few large bodies of fresh water, though they are not on that account of any less interest to the freshwater naturalist. The best-known sheets of water are the Broads of East Anglia. The scientific mind, like Nature, abhors a vacuum and there was no dearth of armchair theories about how the Broads had been formed when Joyce Lambert, a botanist, and J. Jennings, a geographer, set out to collect some facts.

      Dr Lambert pushed her way through the dense fens along straight lines from edge to edge and took borings at regular intervals. At many places the peat was of a different type at different levels, at others it was uniformly of a type that indicated accumulation at the bottom of standing water. In due course an elaborate and plausible explanation of the origin of the Broads was formulated and it might have remained the accepted one for a very long time, a great deal of hard work having gone into the collection of the evidence. However, Dr Lambert thought it prudent to continue her borings, and turned up evidence which demolished the theory. She found peat composed of different plant associations at different levels and peat that had accumulated uniformly in water so close together that the plane between them must be vertical; indeed there was evidence of columns of the former surrounded by the latter. There could be no explanation of this except excavation by human agency and the research became primarily historical.

      No direct evidence has been found but the circumstantial evidence is convincing.

      Documents of the thirteenth and earlier centuries refer to turbary rights in the region of the Broads, and there are records of much peat-cutting in parishes where there was no source of peat other than the fens where the Broads now are. There is no reference to water. After about 1350 there are few references in old documents to turbary, but frequent references to fisheries. There is, therefore, good reason to believe that the Broads are old peat-cuttings which became flooded between 1300 and 1350 probably as a result of some change in the relative levels of land and sea (Ellis, 1965).

      A river tends to build up a deltaic plain at the end of its course and it inundates this plain every time it rises a little above its normal level. Parts of the plain will be under water only at the height of a flood, parts will be permanently marshy, and parts will be under water all the year round. This is the normal and accepted state of affairs in regions of the world where man has done little towards controlling and taming nature: the Rivers Tigris and Euphrates may be taken as an illustration (Fig. 6). In Britain, however, man has long since decreed that there is a place for everything and the place for water is within well defined banks; any breaking out and overflowing is an irregularity and often a catastrophe, and the victim of a flood is not consoled by the assurance that it is “natural”.

      Fig. 6 Lower courses of the Rivers Tigris and Euphrates

      The East Anglian fens originated when a flat clay-floored valley opening into the Wash was flooded by the sea after the Ice Age, owing to a slight lowering of the land level. Silt banks deposited by the sea gradually cut it off and it became a great inland lagoon. It was shallow, and rich in nutrient salts. Conditions were, therefore, good for plant growth and the resulting vegetation was luxurious. The dead remains accumulated and formed peat which filled up the lagoon rather rapidly, speaking in geological terms, till open water was left only in a few meres, which must have been very like the Broads today. Man coveted the rich soil and in the seventeenth century he successfully started drainage and reclamation. Now the meres have gone, the natural vegetation is to be found only in a few carefully tended preserves, and the fenland presents to the pond-hunter no more than an endless series of ditches, great and small.

      Travelling a little farther south, we come to the chalk region; and a more waterless expanse than a chalk down cannot be found anywhere in the country. But even here there is something to interest the freshwater naturalist. Man has been wont to run stock over the downs for centuries and, in order СКАЧАТЬ