Britain’s Structure and Scenery. L. Stamp Dudley
crack is widened. This is the basis of frost action, through which great blocks may be split off from mountains and fall to lower levels as screes. Wind, too, plays its part by blowing away the finer dust and sand whilst strong wind armed with sharp sand particles is a powerful abrading agent. In newly formed mountain areas gravity itself plays a large part—for example in the formation of screes. Both in mountain areas and at lower levels landslides are by no means unknown. Gravity also causes the well-known phenomenon of soil creep, whereby soil gradually slides downhill. The process is seen at work in Plate 9B. Rain collects together to form mountain torrents which in turn unite to form swift rivers sweeping masses of debris always from higher to lower levels, from the land towards the sea. The eroding and transporting action of running water is paralleled in colder climates by the action of moving ice—glaciers which move slowly but inexorably down valleys or great icesheets which ride over the whole surface of the land, scooping out hollows where the rocks are soft, smoothing and polishing them where they are hard. In tundra lands the sub-soil remains permanently frozen whilst the surface thaws in summer and, where there are steep slopes, masses of sludge slide downhill, the whole process being called solifluction. On the margins of the seas and oceans wave action is a powerful force in wearing away the newly formed lands.
Whilst the major surface features of Britain owe their origin to the mountain-building movements of the past and to the character of the rocks which make up the land masses, many of the most striking scenic details are the result essentially of the different processes of weathering on varied rocks. In high mountain areas frost plays a large part and accounts for the angular rock surfaces such as those seen on Striding Edge (Plate XVIB) or in Snowdonia (Plate 8A) or on Cader Idris (Plate XXIX). Sometimes the sculpturing action of frost produces fantastic results, as in the well-known Sphinx Rock on Great Gable in Lakeland. Screes of fallen angular blocks and fragments of rock, most of them broken off by frost action, are a well-known feature in all mountain areas and sometimes dominate the landscape. Plate 30B shows the famous screes on the south side of Wastwater. Blocks of rock dislodged by the undercutting action of the sea and the action of rain form screes along many sea cliffs; a typical example from Cornwall has been shown in Plate 8B to illustrate the angle of rest assumed by loose rock of average character. The angle is much lower where rocks such as clay-shales become slippery when wet, and is lowest where the actual rock may “flow” when wet, which is the case with clay.
Onion weathering under the influence of the sun leaves hard, rounded cores of rock. In tropical countries, these may be almost true spheres; in this country such “cores” scattered over the country are familiar in many granite areas. A good example may be seen on Crousa Common (The Lizard, Cornwall), whilst the interesting weathering of granite, seen in such “tors” as those of Dartmoor (Plate XXVII) is to be ascribed mainly to the same action.
The most interesting results are seen where the original rock varies in hardness. A sandstone, for example, may be indurated along certain lines and the denuding agent whether wind, rain, running water or the sea finds out the pockets of softer sand and scoops them out. The interestingly fretted rock shown in Plate VI is actually the result of the action of the sea, but a very similar appearance might be due to wind action. Where a rock is fractured rain washes out the loose, crushed rock and produces striking cliffs such as those shown in Plate IB. Even in Lowland Britain the “High Rocks” of Tunbridge Wells are simple examples of differential weathering.
Immediately after a great earth-building movement the deposits which fill the hollows—the tectonic valleys and basins—are coarse and often consist of angular blocks which are actually screes and may become consolidated to form a “breccia.” Beds of roughly rounded boulders and large pebbles may be deposited by swift streams to become consolidated later as conglomerates and pebble beds. Plate VIIIB shows an example from the Lake District of such boulders being swept down by a stream in flood. As time goes on the mountains are worn down, yield less material and the beds laid down in the basins and seas become finer grained in character—sands and silts and muds, which may become consolidated respectively into sandstones, siltstones and shales. In the later stages of the cycle muds and clays will definitely predominate and when the lands have been worn down almost to plains (called “peneplanes” or “peneplains”—Latin: pene, almost) they will yield so little sediment that the waters of the surrounding seas may become quite clear. These conditions of clear tranquil water are those under which corals flourish and also other organisms which build up their hard parts of calcium carbonate; thus the deposits then formed are often limestones. The cycle of denudation on the land and of sedimentation in the water is brought to a close by earth movements, it may be slight at first, which herald the oncoming of a new storm. More often the major cycle of events is varied by minor earth movements—it may be the so-called “eustatic” movements, not of folding of the earth’s crust, but of the gentle elevation or depression of blocks of it relative to the level of the waters—so that minor cycles of sedimentation occur within the major. This is well illustrated in the geographical evolution of the British Isles.
So far nothing has been said regarding what is now known of the structure of the earth as a whole. It cannot be too forcibly stated that the old concept of a solid crust, rather like the skin of an apple, covering a molten interior, is entirely wrong and that the simple deduction that the whole was cooling and contracting so that wrinkles—which were the mountain ranges—were being formed just as when an apple dries is equally false. We now know that there is a central sphere, solid and very heavy and probably consisting of an alloy of iron and nickel—thus agreeing in composition with some of the meteorites which from time to time fall on the earth’s surface. This iron-nickel core accounts for the magnetic phenomena of the earth. Enveloping this is the crust, in all about 700 miles thick—a figure which may be compared with a height of 5 miles for the highest mountain and a depth of 6 miles for the deepest ocean. It is well known that there is a rapid increase in temperature as one goes downwards in the crust so that even in a deep mine it is almost unbearably hot.
FIG. 6.—Diagram of the Fault shown in Plate IA. This is a typical example of a very small normal fault. The fault plane separates the downthrow side on the right from the upthrow side on the left. The angle which the fault plane makes with the vertical is the hade; the vertical displacement (here only a few inches, though in big faults it may be thousands of feet) is the throw. Normal faults occur under tension whereas thrust faults and structures such as are shown in Fig. 72 occur under extreme compression.
It does not necessarily follow that the solid core of the earth is extremely hot, since it is now known that heat accumulates in the lower layers of the crust through radioactivity. What is important is not the temperature of the central core but of the crust. At no great depth the temperature must be such that all rocks would be molten were they not kept in a solid or more probably a plastic condition by the pressure of the solid rocks above. Towards the end of a major cycle of denudation, however, so much material has been removed from one part of the surface of the crust to another that the pressure is lessened over the land. Some of the underlying heated layer becomes actually molten and seeks to find weak spots or lines in the crust through which it can escape. It may reach the surface and be poured out through the craters of volcanoes (volcanic eruptions) or through cracks in the surface (fissure eruptions) as lava. Some of the molten rock does not reach the surface but forces its way into cracks and there consolidates as wall-like masses or dykes; or it may force its way parallel to the bedding planes of sediments to form sills. A striking example of an old volcano with associated sill is found in Arthur’s Seat, Edinburgh, shown in Plate XVIA. In all these cases the molten rock bakes and hardens the rocks through which it passes—it changes their form by its contact (Greek: meta- change,
morphe form, hence the process is called contact metamorphism).
FIG. 7.—Diagrammatic Section of an Unconformity A—B is the plane of the unconformity. After the deposition of the group of beds marked C they were gently folded by earth-building movements and were subjected to denudation. Gentle subsidence followed so that the group of beds marked D were deposited gradually over a larger and larger area—they rest unconformably on the