Britain’s Structure and Scenery. L. Stamp Dudley
but to cut them deeply and so one gets the interesting and picturesque feature of incised meanders with a river winding in a gorge, it may be of considerable depth. If in such a case a meander is cut off one gets between the abandoned course and the new course a “meander core.” Incised meanders tend to develop where the rocks are relatively hard. Where a broad valley is excavated in relatively soft rocks the rejuvenated river develops for itself a new alluvial covered flood plain at a lower level than the old one and so fragments of the old one are left as gravel-covered or alluvium-covered terraces. Successive uplifts produce successive terraces at several levels. Those of the Findhorn in Scotland are well shown in Plate XXXI. The terraces of the Thames are not only well known but have been and are very important economically—for the dry sites they offer for settlement, for the water supplies once afforded by the gravels, for the excellent well-drained soils to which they give rise, for the brickearth they formerly supplied for brick making, and latterly for the supplies of gravel which, alas, is being excavated regardless of the future use of the devastated land. In the case of the Thames near London it is possible to distinguish one gravel-covered terrace at about 100 to 120 feet above present sea-level, though naturally varying in height with distance from the sea. This is the Boyn Hill Terrace and is very clearly marked in several areas. There is another terrace, of wide extent, at about 50 feet above sea level known as the Taplow or Middle Terrace. A third one is the Low or Flood Plain Terrace at some 10 or 15 feet above sea level. Then there followed a time when the Thames was lower than at present—or rather when the sea-level was lower and the river excavated what is now a buried channel so that to this extent the estuary of the Thames is a drowned valley. Actually the history of the Thames is much more complex than this, and such a complex history is typical of British rivers. Each change has some corresponding effect on tributaries. In the lower courses of a well graded river the effects of hard bands which may cross the valley have been eliminated and an interesting feature is found when the course of a rejuvenated river is followed upstream. There is found to be a point where there is a break in the longitudinal profile of the river. This is where it is still cutting back as a result of the change in level. Such a break of slope is known as a “knick point” and its development is to a large extent independent of any differences in the rocks of the river bed.
It must be remembered that the British Isles had a well developed river-system before the oneoming of the ice sheets of the Great Ice Age and that the effect of glaciation was to modify rather than to change completely the existing valleys and land forms.
A number of the plates in this book illustrate a few of the extraordinary complex character of British rivers. A drowned estuary such as that shown in Plate 26 may become silted up and a marshy plain may result—well seen in the estuary of the Glaslyn in Plate 20. Rejuvenation may result in gorges even in the middle courses of rivers—as shown in Plate 2. A mature landscape with well-rounded hills affected by rejuvenation is often more apparent from the air than on the ground and an example from the Southern Uplands is well shown in Plate XXII. The interesting case of “drowning” exhibited by the Norfolk Broads is shown in Plate XXIII.
THE WORK OF THE SEA
THE extraordinarily varied character of the sea coasts of Britain and the variety of habitats which they afford to both plants and animals, with the consequent enrichment of our fauna and flora, give a special interest and importance to the story of the work of the sea in the building of the British Isles.
It is now generally agreed that ocean currents play but a very small part in the erosive and transporting work of the sea and that the effects of tidal movements are limited to a few special cases—notably tidal scour in confined estuaries and straits. The work of the sea is primarily through wave action—to some extent through the hydraulic forces engendered by the movement of great masses of water, but far more through the arming of the waves with quantities of rocks, stones, gravel and sand.
The waves of the sea are primarily wind-waves ; they are caused by the disturbance of the surface by wind but, once formed, waves may travel far beyond the area where they were generated—hence “swell” or “ground-swell” unaccompanied by wind. It is, of course, well known that there is no forward movement of the water in wave action, except where the waves are breaking on the coast. The vertical range of motion, in other words the height which waves may reach, is commonly much exaggerated. Waves which are as much as 50 feet from trough to crest are decidedly large, probably quite exceptional even in the open ocean. At a depth of 100 feet the water is little disturbed, at a depth of 500 feet it is doubtful whether there is enough movement to disturb even the finest mud. There is thus a fundamental difference between sub-aerial denudation, which takes place at all heights from sea-level to the tops of the highest mountains, and marine denudation which acts on a very restricted vertical plane above or below the surface level of the water, The maximum effect is where sea meets land—between the tide marks and just above or below.
Consider what happens at the base of cliffs. Angular blocks of rock and stones fallen from the face of the cliff are picked up by the waves and hurled against the base of the cliffs which they thus tend to undercut,
FIG. 18.—Sections showing the Formation of Cliffs These sections illustrate the plane of marine erosion (see Plates 3 and XX A) and the way in which the cliffs are cut back and a submarine peneplane formed.
much in the manner of coal-cutting machinery at the base of a coal seam. Blocks from above then split off along joints and fall by the force of gravity ; where there is a dip of the rocks seawards great masses may slide down the bedding planes. The latter effect is well seen where rock overlies clay the surface of which becomes slippery and acts as a greased plane—hence the constant slips along the south coast of the Isle of Wight and between Dover and Folkestone, in each of which cases chalk overlies gault clay. Plate V shows the famous under-cliff, west of Ventnor in the Isle of Wight. On the shore between the tide marks the rock fragments are rolled against one another and quickly reduced to rounded boulders, pebbles and sand. These, rolled backwards and forwards between the tide-marks and later dragged below low tide-mark enable the sea to level off its wave-cut platform. This is illustrated in Plate 3. The particularly interesting case of undercutting of massive limestone shown in Plate XXA is partly due to the small tidal range and the consequent concentration of erosion along one plane.
Thus the effect of the sea round the coasts may be described as the
FIG. 19.—Section through a Raised Beach. This is a diagrammatic representation of the scene shown on Plate XI
creation of a platform, a wave-cut rock bench, on which is distributed a veneer of sediments made in the process. The process of its development is shown diagrammatically in Fig. 18. This shelves gently seawards under the water and passes imperceptibly into what is called the Continental Shelf. This is a great shelf found round most of the lands over which the sea is less than 600 feet deep.
The surface of the continental shelf is, normally, very gently undulating through relative resistance of the solid rocks. It is, in fact, a peneplane formed by the work of the sea. Even the slightly irregular denuding action of the sea may result in swellings of the floor which just give rise to shallow areas or may reach the surface as islands. Just as the sea in cutting back a cliff may leave a stack or an island, so in the age-long process of marine peneplanation certain upstanding masses may have been left as islands—it may be isolated and far from land. Where this is the case there is usually an explanation in the hardness or resistance of the rocks of which they are composed. The Scilly Isles are thus the protruding surfaces of an almost submerged granite mass comparable with that of Land’s End and from which the surrounding sedimentary rocks have been removed. The isolated mass of Rockall far away in the Atlantic off the north-west coast of Scotland consists of a particularly tough micro-granite and the same is true of Ailsa Craig near the entrance to the Firth of Clyde south of Arran. The celebrated