[BOOK I]
PREHISTORIC LONDON
[CHAPTER I]
THE GEOLOGY
By Prof. T. G. Bonney, F.R.S.
The buildings of a town often succeed in masking the minor physical features of its site—irregularities are levelled, brooks are hidden beneath arches and find ignominious ends in sewers; canals, quays, and terrace walls may be wholly artificial. To realise completely the original contours of the ground is often a laborious process, demanding inductive reasoning on the evidence obtained in sinking wells, in digging the foundations of the larger buildings, or in making cuttings and tunnels. Still the broader and bolder features cannot be obscured, however thick the encrusting layer of masonry may be. What, then, are these in the case of Greater London? Its site is a broad valley, along the bed of which a tidal river winds in serpentine curves as its channel widens and deepens towards the sea. On either side of this valley the ground slopes upwards, though for a while very gently, towards a hilly district, which rises, sometimes rather steeply, to a height of about 400 feet above sea-level. Towards the north this district passes into an undulating plateau, the chalky uplands of Hertfordshire; on the south it ends in a more sharply defined range, which occasionally reaches an elevation of about 800 feet above the sea—the North Downs. The upland declines a little, the valley broadens towards the sea, as the river changes into an estuary. Between the one and the other there is no very hard and fast division; the ground by the side of each is low, but as a rule by the river it is just high enough to be naturally fit for cultivation, while by the estuary it is a marsh. But as the ground becomes more salubrious, the channel becomes more shallow, and at one place, a short distance above the confluence of a tributary stream from the north—the Lea—this change in the character of the valley is a little more rapid than elsewhere.
These conditions seem to have determined the site of the city—the original nucleus of the vast aggregate of houses which forms the London of to-day. Air and water are among the prime requisites of life; no important settlement is likely to be established where the one is insalubrious, the other difficult to obtain. Thus men, in the days before systems of drainage had been devised, would shun the marshes of Essex and Kent, and, in choosing a less malarious site, would seek one where they could get water fit to be drunk, either from brooks which descended from the uplands, or from shallow wells. These conditions, as we shall see, were fulfilled in the site of ancient London; these for long years determined its limits and regulated its expansion.
Let us imagine London obliterated from the valley of the Thames; let us picture that valley as it was more than two thousand years ago, when the uplands north of the river were covered by a dense forest, and the Andreds Wald (as it was afterwards named) extended from the Sussex coast to the slopes of the Kentish Downs. We gaze, as we have said, upon a broad valley, through which the tidal Thames takes its winding course, receiving affluents from either side. These are sometimes mere brooks, sometimes rivers up which the salt water at high tide makes its way for short distances. The brooks generally rise among the marginal hills; the rivers on the northern side start far back on the undulating plateau; but on the southern the more important have cut their way completely through the range of the North Downs and are fed by streams which began their course in the valley of the Weald. Of the latter, however, probably not one is directly connected with the earliest history of London; of the former, only the Fleet, which, rising on the southern scarp of Hampstead Heath, ultimately enters the Thames near Blackfriars Bridge. But both the one and the other at later stages in the development of London may require a passing word of notice.
What, then, do we see at this earliest phase in the history of the future metropolis? At once we are impressed with the fact that the Thames formerly must have flowed in a channel broader but straighter than its present one; a channel which is now indicated by a tract of alluvial land a few feet below the general level of the valley, and but little above high-water mark. Traces of this may be seen here and there between Chelsea and London Bridge, in the low ground about Millbank and along the river-side at Westminster, and in that which runs from Lambeth along the right bank of the Thames. But the most marked indication of this alluvial plain begins about a mile below London Bridge. Here the left bank of the river is formed, as it has been from the bend at Hungerford Bridge, by a terrace ranging at first from about 25 feet to 40 feet above mean tide level, a most important physical feature, for it determined, as we shall see, the site of London. But on the opposite shore, the strip of lower ground—often about four or five hundred yards wide—on which river-side Southwark is built, continues until, at Bermondsey, it widens rather suddenly to about a mile and a half. So it goes on past the junction of the Lea, now widening, now contracting slightly, as, for instance, opposite to Purfleet and above Greenwich, but always a broad lowland through which the present tidal river takes a wholly independent and sinuous course. This plain is formed of silt which the Thames itself has deposited over the debatable region where river and sea begin to meet. It is but little above high-water mark; much of it less than a dozen feet above ordnance datum. A similar low plain, about half a mile wide, may be traced for a few miles up the valley of the Lea, and indications of this may be found here and there by the side of the Thames above Chelsea; but commonly they are wanting, and always are limited in extent. In their absence, the river bank is higher, for it is formed by the scarp of that terrace to which we have already referred. The difference in elevation between these plains is not great, for the second begins at about 20 to 25 feet above ordnance datum; but it shelves from this gently upwards, forming the remainder of the more obvious bed of the valley, till it reaches a height of about 100 feet. At about this level, though it is impossible to be quite precise, the steeper slopes, more especially on the northern side of the river, and the hills, continue to rise till sometimes—as at Hampstead and at Highgate—they reach an elevation of rather over 400 feet. We cannot, however, do more than speak in general terms, for in a valley like that of the Thames—mainly excavated in a soft and tenacious clay—a large part of the rainfall runs off, forming numerous brooklets and small streams, which carve out many minor undulations and shallow ramifying valleys.
The lowest alluvial plain, in olden days, must have been a desolate marshland, the haunt of wildfowl and the home of ague; so we pass it by, to describe more particularly the ground which overlooks it. The valley as a whole—in the neighbourhood of London—is carved out of strata assigned by geologists to the earlier part of the Tertiary era, the period called the Eocene. These rest upon a mass of chalk some 650 feet thick beneath the junction of Tottenham Court Road with Oxford Street—which rises to the surface towards the Kentish Downs on the one side and in the Hertfordshire hills on the other. Near London this rock is not exposed, but it begins to show itself near Deptford and Charlton, and is yet more conspicuous about Dartford and Purfleet, so that it evidently forms a true basin beneath London. Of what lies beneath it we shall speak hereafter, for this is a matter of more importance to the future history of London than at first might be supposed. The Eocene strata take the same basin-like form as the underlying chalk, so that while the lowest of them rises to the surface north and south of London, it is rather more than 200 feet below sea-level at Hungerford Bridge. This rock, called the Thanet Sand, is a marine deposit; it is a very light grey or buff-coloured sand, formed almost entirely of quartz grains, and it occupies a more limited area than the overlying strata, its thickness beneath London commonly varying from about 20 to 40 feet. The Thanet Sand seldom, if ever, reaches the surface to the north or the south-west of London, but it may be seen to the south-east, as about Woolwich and Croydon.