Britain’s Structure and Scenery. L. Stamp Dudley
if they contained different sets of fossils the inference was that they were of different ages. If on the other hand a sandstone from one region contained fossils identical with those from a shale in another the inference was that the two rocks were being formed at the same time—that they were of the same geological age or “synchronous” and could be “correlated.” It was found that when a species died out or “became extinct” it did not reappear in later rocks. Of course this was quite consistent with the idea of a succession of separate creations and it was not till much later that the theory of evolution permitted the tracing of the relationship between the fossils of one set of rocks and those of another.
The determination of the geological age of rocks by the fossils they contain is one of the two fundamental principles underlying the whole study of historical geology. The other is the Law of Superposition. Where one bed of rock rests upon another it is presumed that the upper bed was laid down after the lower and hence that the upper bed is the younger. A large proportion of the fossil-bearing rocks are sedimentary rocks (i.e. they were laid down as sediments under water—in the sea or in fresh-water)—and this law is true for nearly all such rocks. It is also true for streams of lava poured out from volcanoes or associated beds of ashes. Pompeii was there before the ashes by which the city was buried. But the Law of Superposition only remains true so long as the original order of the rocks has remained unchanged. With earthquakes and mountain-building movements the original order may be changed—the rocks may be folded, or even bent right over so that the original order is reversed. But reversal of the order in this way is the exception, not the rule, and can be detected by detailed survey. No one who has spent a holiday on the magnificent coast of north Cornwall can fail to have noticed how folded and broken are the rocks exposed along the sea cliffs. Examples are shown in Plates III and IV.
Presuming the original order to have been maintained, if the upper bed in one locality be traced laterally it may be found to pass under still higher beds in another so that the higher beds in the latter area are still newer. In this way a whole succession of strata may be built up—from the very oldest at the bottom to those still being formed at the top. Such a succession has, in fact, been constructed as a result of patient research and forms what is sometimes called the geological column. It must be realised that the rocks of the geological column are not to be found complete in any one area.
It must not be thought that if a hole were bored in the earth’s surface it would pass through all the rocks shown in the geological column. At the present day deposits of sand, silt and mud are being formed in the shallow waters near the estuaries or deltas of the great rivers of the world while other deposits, some of them consisting mainly of the hard parts of organisms living in the water, are being formed over the floors of most of the seas and oceans. In the lakes of the world other deposits are being laid down and even on parts of the land surface deep layers of sand and dust brought by wind are being spread over the older rocks. This is quite clearly seen where, as in the Culbin Sands of Morayshire shown on Plate XII, sand dunes are burying growing vegetation. These are all areas where deposition is taking place and where new strata are being formed. But over most of the land the rocks are being worn away by the combined action of rain, wind, sun, frost, running water and moving ice and its surface slowly but inevitably lowered. These are areas of denudation (Latin, denudo, I lay bare) and to them may be added the margins of the oceans where waves beat against the shores and wear them away. There will be no deposits in such areas to mark the present day and it was the same in the past. Thus beds present in one locality may be absent in another so that in the latter place there is a gap in the succession. Such a gap may indicate that the region concerned formed part of a land mass at the time in question or came otherwise under the influence of denudation. When the region again sank below sea-level and strata were again deposited it was perhaps after a lapse of many millions of years. Here is a “stratigraphical break” between the older and younger rocks. The younger rocks are said to rest “unconformably” on the older in those cases where the older had been folded and denuded in the meantime. A typical unconformity is shown in Plate IVB.
There is another difficulty in reconstructing the stratigraphical column. When one bed of rock or stratum is traced laterally it may change its character and unless the whole change can be traced and a limestone, for example, found to pass laterally into a shale or sandstone, it may be difficult to say that the limestone in the one place is of the same age as the sandstone in another. Of course if both types of rock contain the same fossils the answer is easy, but just as different habitats at the present day—muddy waters and clear lime-rich waters—may not have even a single species in common, so it happened in the past, and the fauna of a limestone may be completely different from the fauna of a bed of shale of the same age. To take a specific example, the beds known collectively as the Old Red Sandstone were laid down in freshwater lakes at the same time as the marine beds of the Devonian were being deposited elsewhere. In such cases the rare instances where the faunas are mixed, or there are “marine bands” representing incursions of the sea in the midst of a fresh-water succession, are invaluable in establishing the essential correlation. Thus the evidence which the geologist has to piece together is at the best fragmentary: it is rarely too that the rocks he wishes to study are “exposed” over large areas. In a country such as Britain the surface is hidden by soil and vegetation and only in some of the higher mountainous areas or along sea cliffs do the bare rocks outcrop at the surface. Elsewhere the geologist has to seek his evidence in quarries, mines, railway cuttings, well-borings, casual excavations for drains and sewers and even in some cases may be faced with the necessity of opening up a special pit in a crucial spot.
The rocks which are seen in the Stratigraphical Column were deposited over an immense period of time. Time is continuous, but there are certain natural phenomena which serve to divide it into definite units. The phenomenon of day and night serves to define one unit of time—the day ; the movement of the earth on its orbit round the sun defines another—the year. Larger units than the year are difficult to define but just as the astronomer uses a “light-year”—to define an enormous distance, so the geologist needs a larger unit than the year. The historian frequently takes the time between two important events to define a period ; thus when we talk of Tudor Times we mean the period when the Tudor kings were on the English throne, though we are able to define this period accurately in years—from the accession of Henry VII in 1485 to the death of Queen Elizabeth in 1603. The prehistorian is no longer able to measure his periods so accurately: he is obliged to define them in terms of the works of man in the periods concerned. The geologist, in his turn, has to deal with the vast periods of time which elapsed before the appearance of man on the surface of the earth ; for the definition or delimitation of such periods the year is an inadequate unit. No one would hand a traveller going on a long sea voyage a six-inch ruler and ask him to measure thereby the distances between the ports en route. Yet the voyager, by careful observation of time and direction, might be able to give a very fair account of the relative positions of the points touched, a close estimate of the distances between them and a good general account of their chief features. It would depend on his power of accurate observations and of using all the available evidence in its appropriate place. Thus the geologist has built up a good general picture of the evolution of the earth’s surface, a picture which is continually gaining in accuracy, and the geological time-scale is divided into a few great eras and a number of periods. The smallest unit of geological time is the hemera, usually named after a dominant animal or plant which was living at the time. A difficulty is that the animal or plant may have been local in its distribution, so that its absence from the sequence in a given locality is scarcely sufficient evidence that no deposition of beds was going on there at that time. A somewhat larger unit is the zone which, though named after a characteristic fossil, is usually to be defined by a characteristic associated series of fossils. A number of zones normally comprise a formation of rocks, while several formations make up a system of rocks. Thus we talk about the Chalk and the Lower Greensand as two of the formations in the Cretaceous System of rocks. But the word system refers to the rocks in the geological column: the rocks in a system were laid down in the period of time known as a geological period so that the measure of time concerned in this case is the Cretaceous Period. A number of periods are included in each of the four great eras into which geological time has been divided since the general appearance of life on the surface of the globe—i.e. since