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.
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.