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EARTH HISTORY—TIME AND LIFE

WE ARE fortunately living in one of the quiet periods of the earth’s history, though not at a time when it has been quiet for so long that the surface has been worn down to a dull and monotonous level. Although the accurate instrumental observation of earthquake phenomena demonstrates that the earth’s crust is rarely absolutely still, the occasional earthquakes which do occur, however severe they may be, are in reality but very gentle reminders of those periods in the history of the earth when the whole surface must have been torn by the most violent and long-continued cataclysms which bent and folded and broke the hardest rocks; which caused whole blocks of the earth’s crust to be thrust many scores of miles over other blocks; which caused continents to sink below the waves of the sea, or which caused vast tracts of the ocean bed to be raised to form the world’s highest mountains. The earthquakes of to-day are like the final murmurs of a great storm which has passed. Even they tend to occur in certain defined earthquake belts and are reminders that the earth’s crust has certain lines and zones of weakness whereas other parts are relatively stable.

If we exclude the great mountain-building movements which took place in the dim early days of the earth’s history—in the Pre-Cambrian—there have been three great periods of mountain building or “orogenesis” (Greek, oros mountain; genesis origin, creation) as far as Europe is concerned. Each of these has played a major part in determining the present-day features of Britain. These three great mountain-building periods were:

(a) at the end of the Silurian and beginning of the Devonian periods—the Caledonian earth-movements, so called because they built up the great mountains which have since been worn down to form the Highlands of Scotland (Caledonia);

(b) at the end of the Carboniferous and beginning of the Permian periods—the Armorican or Hercynian earth-movements, so called because they caused the great folding of the rocks seen in Brittany (Armorica) and the Hartz Mountains of Germany. In Britain they caused the uplift of the Pennines, the Malvern and Mendip Hills and folded the Coal Measures into basins.

(c) in the middle of the Tertiary era—the Alpine earth-movements, so called because they were responsible for the rise of the Alps as well as of many of the great mountain chains of the world to-day but which affected Britain much less than the previous movements.

Of the still earlier earth-movements at least one left very important marks in Britain—the Lewisian, which caused the folds in the rocks in the extreme north-west of Scotland and which may have been contemporary with the folding of the ancient rocks which peep from beneath a cover of later strata in the Charnwood Forest of Leicestershire.

In each case the mountain-building movements gathered strength slowly as with a developing storm and gradually reached a peak when the whole earth must have experienced a constant succession of gigantic earthquakes. Then gradually they must have died away again, the whole cycle stretching over an immense period of time. The result of these earth-building or orogenic movements was to form a series of gigantic wrinkles in the crust of the earth—these are the main mountain chains—between which are broad areas but little disturbed—the “tectonic” basins (Greek: tektonikos, related to building—i.e. not formed by later excavation. Sometimes these basins were below sea-level and became the areas of sedimentation in the succeeding periods, while the surrounding mountains as soon as formed were attacked by the forces of denudation which started to wear them down. So we get the idea of the geological history of the earth moving in great cycles. The first is what may be called the major cycle of denudation. This may be considered to begin when earth movements have caused land to rise above the level of the waters in the surrounding ocean. No sooner does this happen than the forces of sub-aerial denudation get to work. The heat of the sun heats the rocks and the different minerals of which they are composed have differential rates of expansion so that, especially with nightly cooling, the rocks are disrupted and a peeling or exfoliation (Latin: folium, a leaf) by successive layers takes place. This is sometimes called onion weathering and is well seen in hot dry countries at the present day. The direct action of the sun is called insolation.

FIG. 5.—Diagrams showing the mechanism of exfoliation or onion weathering of rocks under the sun’s heat

Falling rain has a direct mechanical effect in washing away the finer particles, a less direct effect by dissolving some of the less stable minerals and an indirect effect by soaking into crevices. There it may be frozen and the water in changing to ice expands so that the 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 older series and at the same time overstep them. In the centre of the basin fine-grained shales were deposited and the diagram suggests that sedimentation was almost continuous. Towards the margins of the basin the fine-grained deposits pass laterally into sands and other coarser sediments and to shore deposits.

This section represents diagrammatically the relationship between the Silurian and the underlying Ordovician in the Welsh Borderlands. See also Plate IV B.

Thus clay and shale are baked into hard slatey rocks, limestone is changed into marble. Some of the molten rock is very fluid when it is first poured out and spreads evenly over a wide surface as did the basalts of Northern Ireland; sometimes it formed hexagonal columns on cooling as at Giant’s Causeway and the Island of Staffa (Plate XVA). In other cases the molten rock was very sticky and consolidated almost on the spot—the famous conical “spire” of Mont Pele of Martinique in the West Indies is the best modern example of this (it was formed during the disastrous eruption in 1902) but there are many examples from earlier periods in the British Isles, such as Ailsa Craig in the Firth of Clyde. Even more important, though it cannot be observed at the surface, is the underground movement of great masses of molten “magma.” At the height of great earth-building movements the magma is squeezed into the core of mountain ranges so that millions of years afterwards, when the mountains have been denuded down to their roots, this core is exposed. A typical rock so formed is granite. The metamorphism caused by a huge granite mass taking eons to cool can perhaps be imagined rather than described and the “metamorphic aureole” is often very extensive; it is economically important because of the valuable metallic minerals which are associated with the gases and heated liquids given off by the magma. These latter often find their way into cracks or veins and there the minerals are deposited—hence the association of ores of tin and copper with the metamorphic aureoles of the granites of Devon and Cornwall. These Devon and Cornish granite masses are the roots or cores of giant mountains formed by the Armorican earth movements but long since worn down almost to a level surface. It is clear that there is a definite cycle of igneous activity associated with a cycle of earth movements—volcanic activity heralding the oncoming storm; intrusion and movement of huge underground masses at the height of the storm; and finally renewed volcanic activity when the storm is dying away. The few volcanoes on the surface to-day which are active may be regarded as the last remnants of the once wide-spread activity at the end of the Alpine earth movements. Many of those are at the end of their lives—dormant or even extinct or merely giving off vapours (“sol-fataric stage”). Hot springs and geysers are indications of a nearly dead volcanic area.

One of the still unsolved puzzles of earth history is whether or not there have been true climatic cycles in the past. There is no doubt that at several periods there have been ice ages, though perhaps nothing as severe as that which overwhelmed the northern hemisphere so recently in geological time that man was already well established and hence known simply as “the Ice Age.” We know definitely that some of the red rocks found in the British Isles—such as the Permian or New Red Sandstone—were laid down under desert conditions and there must have been other times when what is now our country must have been wet and hot. Many of the phenomena, however, may be explained by a different distribution of land and water in the past or at most by a shifting of the earth’s axis.

FIG. 8.—Major episodes in Earth History

FIG. 9.—Some characteristic Fossils.

Those numbered I are Palaeozoic; those numbered II are Mesozoic and III Cenozoic.

Graptolites:—Ia Monograptus (Silurian); Ib Diplograplus (Ordovician); Ic Didymo-graptus extensus (Ordovician); Id D. murchisoni (Ordovician). Trilobites:—Ie Phacops caudatus (Silurian); If Calymene blumenbachi (Silurian); Ig Ogygia buchi (Ordovician). Primitive fish:—Ih Pterichthys milleri and Ii Cephalaspis lyelli (Old Red Sandstone). Brachiopods:—Ik Spirifer verneuilli (Devonian); IIb Spiriferina walcotti,; 11c Rhynchonella rimosa and 11d Waldheimia numismalis (all Lias). Ammonites:—11a Dactylioceras commune (Lias); 11e Pcrisphinctes biplex (Jurassic, Kimerid-gian); 11f Hoplites splendens (Cretaceous: Gault); 11g Hamites attenuatus (Crctaceous: Gault): 11h Hoplites auritus (Cretaceous: Greensand). Nummulites: 111a Nummulites laevigatus (Eocene).

The existence of a plastic or semi-molten layer under the solid part of the earth’s crust has already been argued and there is nothing inherently impossible in the idea that the continents consist of relatively light rocks and form masses as it were floating on a plastic layer. If this is so, it is but one step towards the idea that the continental masses may drift away from or towards one another—hence the Theory of Continental Drift associated with the name of the German geologist Wegener. But the attempt to secure observational confirmation of drift, however slight, has been disappointing.

The similarity in the rocks which make up such widely separated lands as Africa, Peninsular India and western Australia is so striking, however, that this absence of direct observational proof of drift is inconclusive. It may be that drift only takes place at certain periods when the underlying rocks are in a particular condition of plasticity.

Amidst all the changing scenes which geological time has witnessed—for we may say that geology is really geographical evolution—there has gone on the evolution of living organisms. Just as in times of war things happen and life is speeded up so there is some evidence to show that the rapidly changing environmental conditions which must have characterised the great periods of earth movement induced a rapidity of organic evolution. Whether that be so or not the fact remains that before the Caledonian movements the world was populated almost exclusively by lowly plants which have left few traces and by invertebrate animals. After the movements was the age of fishes—the many weird forms of the Old Red Sandstone—and a rapid evolution of fern-like plants. Before the Armorican movements there were some amphibians but it is after the movements that we have the great age of reptiles and the seas became populated by the well-known coiled ammonites and innumerable brachiopods. The curious graptolites which scarcely survived the Caledonian movements had completely gone. The doom of the heavily armoured and small-brained Jurassic and early Cretaceous reptiles was sealed well before the earliest inklings of the Alpine movements. The victory of the mammals, with man himself to follow later, was assured long before the time the Alpine storm really broke.

FIG. 10.—Diagram illustrating the distribution in time and space of a typical fossil. ZI, ZII, ZIII, ZIV, ZV are zones

The species 1 is found at all five localities A, B, C, D and E and is restricted in its vertical distribution to Zone III. Only in one locality, C, is it found slightly below and slightly above the limits of the zone. It is therefore a good “zonal index.” Species 2, on the other hand, is only found in three of the five localities; it has a wide range in time and is found at a much lower horizon in locality C than in locality E. It is therefore of no use as a zonal index but such a distribution is characteristic of a “facies” fossil—a species seeking some special habitat conditions such as shallow water near a shoreline.

We are now in a position to apply the general principles already enunciated and to see how far they explain the building up of the present-day structure of the British Isles and to see what evidence the rocks of this country contain of the evolution of the great world groups of animals and plants and, in the later stages, of our own particular native fauna and flora.