Life in Lakes and Rivers. T. Macan T.

      Fig. 5 Diagrammatic cross-section of the Mackereth core-sampler (not to scale). The letters are referred to in the text. (Limnol. Oceanogr. 1958)

      Mr Jenkins’ apparatus proved excellent for use on Windermere, where the necessary pontoons could be borrowed, but not elsewhere, and accordingly Mr F. J. H. Mackereth devised a portable model (Fig. 5). The problem was to ensure stability while the core was being obtained. This he solved by basing the corer on a large cylinder resembling a dustbin (G). This sinks some way into the mud when the apparatus has been lowered, and is then forced farther in by means of a pump (P) which removes the water from its upper portion. A secure base has now been secured for the rest of the operation. The core is obtained in a long tube (B) housed inside a second tube (A), which is attached to the centre of the top of the anchoring cylinder. The second problem was how to drive the corer into the mud from a small boat that could not easily be kept exactly above the apparatus. Compressed air passing down a flexible tube (O) was the solution. It involved a piston fitting inside the outer tube and closing the top of the inner one (C). Some means of evacuating the inner tube as it moved downwards was essential, for otherwise a solid cylinder rather than a tube was being forced into the mud. This is achieved by a fine central tube (D) which holds in position a piston (F) at the mouth of the inner tube when this is retracted, passes through the upper piston and out through the top of the outer tube (L) to which it is attached. When compressed air admitted to the top of the outer tube forces the inner tube down into the mud, the air in the inner tube escapes through the fine central tube and the corer passes into the mud, causing no more compression than is due to the friction of the walls. When the inner tube is nearly fully extended, the compressed air escapes into a side tube (I), which leads it into the anchoring cylinder. This is forced out of the mud and brings the whole apparatus to the surface. Compressed air passed into the fine inner tube brings the inner tube back into the outer and, at the same time, ejects the core. This apparatus has been carried to tarns in the mountains by a helicopter and successfully used there.

       CHAPTER 4

      DIFFERENT KINDS OF LAKES

      Lakes, geologically speaking, are transitory features of the landscape. The biologist who studies a lake is likely sooner or later to find that the answer to some problem he is seeking to solve is to be sought in past history, and particularly in the way the lake was formed. Lakes and ponds have originated in many different ways and much ingenuity has been devoted to fitting them into schemes of classification. We shall not dwell on the groupings and subgroupings which have been suggested, but, in the first part of this chapter, prefer to take the lakes as they come, starting in the north of Scotland and travelling southwards.

      The Great Glen is a tear in the earth’s surface, and Loch Ness, which lies in it, provides an example of a lake associated with faulting. Loch Ness is 21³/₄ miles (35 km.) long and a little under one mile (1.6 km.) in average breadth, so it is a long and narrow lake; its greatest depth is 754 feet (230 m.) and its mean depth 433 feet (130 m.), so it is deep and steep-sided. Its mean depth is greater than that of any other British lake by quite a big margin, though there is one, Loch Morar, which is deeper at the deepest part (1017 feet = 310 metres). All these features are characteristic of tectonic lakes, that is lakes formed originally by movements of the earth’s crust. Also in this class are some of the most striking lakes of the world, such as the Dead Sea and the lakes in the Great Rift Valley of Eastern Africa. These were caused partly by lateral tearing, as is the Great Glen, but there followed a lowering of a strip of the earth’s crust so that what is now the floor of the rift valley was once level with the high land on either side.

      Most of the other lochs in the Scottish Highlands owe their present form to the work of ice when the country was covered with it during the Ice Age and so were the llyns of the Welsh mountains and the lakes of the English Lake District. Indeed, nearly all the larger stretches of water in Britain were formed by glaciers, at least in part. In some cases their basins were gouged out by glaciers flowing down mountainsides, usually in valleys cut by a stream in an earlier, more clement period. When the mass of snow and ice and rubble reached the bottom of the slope it dug into the ground and excavated a great trench. This trench became the basin of the lake when the glacier retreated with the onset of warmer conditions. The mass of material which the glacier plucked from the land it passed over was deposited in mounds or moraines at the snout of the 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