Life in Lakes and Rivers. T. Macan T.
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 to provide them with water, he has built ponds which have received the name of dewponds. There are few subjects about which more nonsense has been written. One explanation offered, even by people who should know better, is that dew-ponds are made by a secret process which insulates them from the surrounding land. When heat is lost at night by radiation from the surface, warmth from the lower layers is conducted upwards, and therefore the temperature at the surface does not reach a very low point; but this upward conduction cannot affect the dewpond because it is insulated. Accordingly, it is alleged, the dewpond area gets very cold, the atmosphere above it is chilled and moisture is deposited. The difficulty about this theory is that, if the dewpond were so effectively insulated from the land below, it would get very hot when the sun shone on it by day and much water would be lost by evaporation. Furthermore, considerations of the respective latent heats of water and chalk (that is the amount which a given volume of each would lose in a given time) have been ignored. Several people have examined the problem both experimentally and theoretically and the whole fallacy has been exposed more than once. Mr A. J. Pugsley (1939) has returned to the attack in a small book published by Country Life, but it would be optimistic to expect that the myth has been exploded. There is certainly a secret process in the making of dewponds and it has been handed down from father to son in certain families, but its aim is the construction of a waterproof bottom which will last for many years without cracking.
The dewpond, in effect, is a shallow pan of concrete or clay, and, though sometimes it is situated on top of a hill where it must rely entirely on rain, often it is located to take advantage of storm water, particularly where a road presents an impermeable surface. The belief that dewponds date back to the Neolithic Age is erroneous.
We have now worked our way down to the south of England and come to the coast to study a freshwater lake which owes its origin to sand-banks thrown up by the sea. To the west of Bournemouth lies Poole Harbour, a big enclosed area connected with the open sea by a small entrance, which cuts through a narrow strip of land and so makes two peninsulas. The one which lies to the west is known as South Haven Peninsula, and a conspicuous feature of it is the Little Sea, a shallow lake over 70 acres (28 ha.) in extent. Particular interest lies in the fact that the origin and development of Little Sea can be traced in detail from the information given on old charts. The first of these, dating from the reign of
Fig. 7a Formation of the Little Sea, c. 1600
Henry VIII, is not very accurate, but from it and one or two later charts a fair deduction is that the peninsula then comprised only land of the Bagshot Sand formation, with a small more recent sandspit at the tip. This is shown in Figure 7a, but the sea and the area between tidemarks are omitted from this figure, as any attempt to include them would be based largely on conjecture. A chart of 1721 is remarkably accurate. It shows that a sandbank, thrown up parallel with the land existing in the previous century, has enclosed a lagoon which is apparently a sheet of water at high tide but at low tide an expanse of bare sand, except for water standing in drainage channels. There is a wide beach, shown stippled in Figure 7b, and a detached sandbank lying to the north of the channel draining the lagoon. Rather more than a century later, in 1849, a survey shows considerable development; a second ridge has
Fig. 7b Formation of the Little Sea, c. 1721
been thrown up parallel to the first in the northern half and marshy area indicates the depression between the two; a third is foreshadowed by a long sandbank which now bounds the outflow on the seaward side; it is shown white in Figure 7c, the convention used to denote land above high water, though strictly it should be stippled as, according to the chart, it was covered by the highest tides. The