Over the Thanet Sand comes a rather variable group of clays and sands called the Woolwich and Reading Beds. They extend over a larger area and generally run a little thicker than it does, for beneath London they are usually about 50 or 60 feet, and occasionally rather more. The fossils are sometimes fresh-water forms, sometimes estuarine or marine, so that the deposit probably marks the embouchure of one or possibly two large rivers. Next comes that brownish clay which is so constantly turned up in digging sewers or foundations, especially on the lower slopes of the hills. Its name—the London Clay—is taken from the metropolis; but it covers, or at any rate has covered, a much more extensive area, for it can be traced at intervals (large masses having been removed by denudation in some districts) as far as Marlborough on the west, the Isle of Wight on the south, and Great Yarmouth on the north. The same cause has reduced its thickness in parts of the metropolitan districts. Beneath Trafalgar Square, for example, it is 142 feet, and in some wells even less, but the total thickness must have been—indeed in some places it still is—rather more than 400 feet. At the base a band of pebbles commonly occurs. This, under the central part of London, is inconspicuous; but farther away, especially towards the south and the east, it is often 20 to 30 feet thick, and sometimes more. It consists of well-rounded flint pebbles, generally not so big as a hen’s egg, mixed with quartz sand. This gravel lies at or near the surface over a considerable area about Blackheath, Charlton, and Chiselhurst, and is now generally distinguished from the London Clay by a separate name—the Blackheath or the Oldhaven Beds. Both this formation and the London Clay contain fossils, sometimes rather abundantly, which indicate a marine origin, though the deposit cannot have been formed at a long distance from land, for estuarine species occur in it; while fossil fruits and pieces of wood are sometimes common in the London Clay, the latter being often riddled by the borings of teredines (a bivalve mollusc which still exists and makes great havoc in timber). So that in all probability both the gravel and the clay were connected with the rivers already mentioned.

Above the London Clay comes a group of sands, occasionally containing intercalated beds of clay, which once must have had almost as wide an extent as it, but in the London area it is reduced to isolated fragments, capping the clay hills at Hampstead, Highgate, and Harrow. Here the deposit is less than 100 feet in thickness, for so much has been washed away, but it often reaches quite 300 feet on the dry moorland about Chobham, Aldershot, and Weybridge.

Then comes a great gap in the geological record. The beds just mentioned belong to the Eocene, but after these nothing more is found till we are very near the end, if not actually out of, the Pliocene period. This long interval, in the district with which we are concerned, was occupied by earth-movements, the result of which was denudation rather than deposition. As we have already said, the chalk and the overlying Eocene strata are bent into the form of an elongated basin, which is related to the long dome-like elevation from which the hills and valleys of the Weald have been sculptured. Basin and dome alike are the results of wave-like movements which began to affect a large portion of Europe soon after the latest Eocene deposits in the London area were formed, movements of which the Alps and the Pyrenees are more conspicuous monuments. But these folds first began at a still earlier epoch—that which separates the newest part of the chalk from the oldest beds of the Eocene. Even then the London basin and the Wealden dome must have been outlined, though less boldly than now; for beneath the City the Thanet Sand and the Woolwich and Reading Beds are pierced in borings, and are together about 90 feet in thickness. But high up on the North Downs the pebble bed at the bottom of the London Clay may be seen resting on the chalk. So this district in early Eocene times must have been higher than the former one by at least the above-named amount; or, in other words, the basin of the Thames and the dome of the Weald must have been already indicated.

GEOLOGICAL MAP OF THE SITE OF LONDON

[To larger image.]

The later earth-movements, however, were on a much grander scale. Under London Bridge the base of the London Clay is about 90 feet below the sea-level, while on the North Downs it is about 750 feet above it, so the displacement since it was laid down has been at least 840 feet. The uplift in the central part of the Weald was doubtless much greater, but as denudation must have begun as soon as ever dry land appeared, we cannot say to what height the hills in this part may have risen. Still, when we remember that the valleys of the Wey, the Mole, and the Medway, which drain the northern half of the Weald, have cut completely through the range of the North Downs on their course towards the Thames, and are the makers of the valleys in which they flow, we can understand the magnitude of the work of denudation. But that work was far too complicated, the subject is far too difficult and full of controversies, to be discussed in these pages; we must content ourselves with mentioning a few facts which have more or less affected the history of the metropolis.

As the rivers flowed, they transported and deposited the débris of the land, and if ever a submergence occurred, the same work would be done by tides and currents of the sea. The earliest deposits, obviously, would be formed in places which are far above the present beds of the streams. Most of these deposits would be washed away, their materials would be sorted out and transferred to lower levels, as the valleys were widened and deepened, and as the surface of the ground approached more nearly to its present contours. Thus gravels, sands, and clays are found at various levels down nearly to the present level of the Thames. The oldest of them, deposited within a radius of about ten miles from London Stone, lie rather more than 300 feet above the sea.[1] These last are probably connected with similar sands and gravels which cover considerable areas in the Eastern Counties, and may have been deposited at an epoch when even the outline of the present valley system of the Thames had not been delineated. Upon this question, however, it is needless to enter. The next deposit, supposing these patches of sand and gravel to be of one age—a very doubtful matter,—is the Boulder Clay—a stiff, tenacious clay, often studded with pieces of chalk, from minute grains to biggish lumps, which commonly are fairly well rounded, together with flints, both rounded and angular, and fragments of other rocks. These have been derived, generally speaking, from the north and from various places, often at long distances, either on the eastern side of England, or in Scotland, or occasionally even in Norway. The clay also appears to have been formed from materials which came from the same direction. But little of this Boulder Clay now remains in the neighbourhood of London; the nearest patch is found on the higher ground on either side of the 300 feet contour-line between Whetstone, Finchley, and Muswell Hill—perhaps also at Hendon. To what extent the valley system of the Thames was sculptured when the Boulder Clay was formed; why the latter stops short on the northern slopes; under what circumstances it was deposited—are all subjects of controversy which it is impossible to discuss in these pages. Suffice it to say that the clay indicates, to some extent at least, the action of ice; and that as the patches of it and of the associated gravels occur at different levels (a fact which is still more obvious in other districts) and appear to exhibit a general correspondence in distribution with the present contours of the ground, several valleys must have been by then partially, if not completely, defined. In the main valley—as, for example, near Erith—fine sandy clays or “brick earth” occur, which some geologists consider to be at least as old as this clay. The climate, when this “brick earth” was deposited, certainly must have been much colder than it is at present, for remains of the musk-sheep have been found in it, and at that time the valley of the Thames must have been excavated nearly to its present depth. But on the slopes of this valley, and of its tributaries, beds of gravel are found, containing stones which must have been washed out of the Boulder Clay; and as these gravels often extend to more than a hundred feet above the present level of the river, the changes since they were deposited must have been considerable. They contain the bones of extinct mammals, such as the woolly rhinoceros and the mammoth, with others indicative of a climate distinctly colder than at present. But as these also have been found almost at the present level of the river, the animals must have remained in this country till it had assumed very nearly its present outlines. For instance, the tooth of a mammoth was discovered in 1731, 28 feet below the surface, when a sewer was dug in Pall Mall. Many bones of this and other animals have been found in the “brick earth” of Ilford; and a splendid pair of tusks, obtained in 1864, is now preserved in the British Museum, South Kensington. Here “the ground forms a low terrace, bordering the small river Roding on the one side, and on the other it slopes gradually down to the Thames. The height of the surface at the pits is about 28 feet above the Thames.”[2] It is therefore certain that the river valley was cut down nearly to its present level while the climate was still much colder than it is at present, and very probable that its depth was much increased after the chalky Boulder Clay had been deposited; for these gravels, as has been said, are strewn over the lower slopes up to about a hundred feet above the present river. At Highbury Terrace they even reach 154 feet, and at Wimbledon 190 feet.

These gravels have yielded the remains, not only of the mammoth, but also of man. His bones indeed have hardly ever been discovered, but stones chipped into shape by his hands are far from rare. They are almost always made of flint, a material which was abundant, could be readily trimmed, and afforded a good and durable cutting edge. These implements are never smoothed or polished, and exhibit many varieties of form. They range from mere flakes, the artificial origin of which cannot always be proved, but which in all probability were used as knives and scrapers, to instruments which could only have been made at the cost of considerable time and some skill. Similar remains have been found elsewhere in the more eastern and southern counties of England and on the Continent. The people, however, who fashioned such implements hardly can have been so far advanced in civilisation as the wandering tribes of Esquimaux in Northern Greenland.

These worked flints are very rarely found either below 20 or above 100 feet from the sea-level, but between these heights they are not uncommon. About two centuries ago a well-worked flint, something like a spear-head in shape, was found with an elephant’s (mammoth’s) tusk “opposite black Mary’s, near Grayes Inn Lane.”[3] Implements of various shapes have been obtained from the gravel near Acton, Ealing, Hackney, Highbury, and Erith, as well as at Tottenham Cross, Lower Edmonton, and other places in the valley of the Lea. But the most interesting localities hitherto investigated are in the neighbourhood of Stoke Newington and of Crayford. The worked flints at the former place are found at more than one level, and indicate a progress in manual skill sufficient to lead observers to the conclusion that they belong to more than one epoch. The newest of these implements, flint flakes with occasional more elaborate specimens, were so abundant and have occurred in such a manner as to suggest to their discoverer (Mr. Worthington Smith) that they lay on the actual surface where they were fashioned by the workers of olden time. “The floor upon which this colony of men lived and made their implements has remained undisturbed till modern times, and the tools, together with thousands of flakes, all as sharp as knives, still rest on the old bank of the brook just as they were left in Palæolithic times. In some places the tools are covered with sand, but usually with four or five feet of brick earth.... That (the floor) was really a working place where tools were made in Palæolithic times is proved by the fact of my replacing flakes on to the blocks from which they were originally struck.”[4] At Crayford also, a layer of flint chips was found by Mr. F. C. J. Spurrell in “brick earth” at the foot of a buried cliff of chalk. The circumstances under which these flakes occurred led him to the conclusion that this also was the site of an old “workshop” of flint implements.[5]