2.1 Palaeogene
2.1.1 London Clay
2.1.2 Lower Tertiaries
In the London Basin the Palaeogene beds are grouped as follows:—
Bagshot beds
London Clay
Blackheath and Oldhaven beds.
Woolwich and Reading beds
Thanet Sands.
The lower group of three beds has been defined as the Lower London Tertiaries.
2.1.1 London Clay
The lowest division is formed chiefly of sand, the middle division of clay ; the upper division, like the lower, consists for the most part of sands.
The London Clay fills the basin and covers by far the greater part of the area. Here and there, however, it is overlaid by the Bagshot beds, which form low hills or plateaux rising above the general level. Owing to their sandy character these elevated tracts are by nature barren and open heaths or commons. The largest lies to the north of Guildford and stretches twenty-four or twenty-five miles from east to west, and about twelve miles from north to south. It includes Aldershot Common, Ascot, Bisley, Bagshot Heath, &c. Nearer to London, Highgate Hill and Hampstead Heath are capped by the sands and gravels of the Bagshot series; many other outlying patches occur, especially in Essex.
The London Clay occupies by far the greater part of the basin. It is usually a fine bluish-grey clay, which weathers brown towards the surface. It commonly contains iron-pyrites and crystals of selenite, and also layers of septaria. The last are concretionary masses conconsisting of a mixture of clay and carbonate of lime. In some districts they are the only hard stones available, and accordingly have been used in many of the older buildings ; and they have also been employed in the manufacture of cement.
The London Clay is 400 or 500 feet thick near London, but towards the west it thins and is partly replaced by sands.
Fossils are not generally common. Amongst the characteristic forms are Ditrupa plana, Aporrhais Sowerbyi, Aturia ziczac, Valuta Wetherelli. In the Isle of Sheppey remains of plants are very abundant, and birds, turtles and snakes have also been found. The plants include leaves, stems and fruits of palms, figs, magnolias, &c., and indicate a climate much warmer than that of the present day.
The London Clay becomes sandy towards the top and passes up without a break into the Bagshot beds above. As fossils are rare and the lithological change is gradual, no very definite line can be drawn between the two formations ; and it is by no means improbable that the sands of one locality may be in part contemporaneous with the clay of another.
2.1.2 Lower Tertiaries
Oldhaven, Blackheath, Woolwich & Reading and Thanet Beds
The Lower London Tertiaries form a narrow border around the basin, widening to the east of London into a fairly extensive but very irregular band.
The Lower London Tertiaries as a whole are thickest on the southern margin of the basin, especially in the east; and they thin towards the north.
The Thanet Sands are generally light-coloured sands, but towards the base they become argillaceous and full of glauconitic grains. Where they rest upon the chalk there is usually a layer of unworn green-coated flints. After the sands had been deposited, percolating water continued to dissolve the chalk beneath, and the flints were left behind. The green coating is probably due to the glauconite in the sands.
The Thanet Sands are well developed in the Isle of Thanet, but they thin towards the west and towards the north. The fossils which they contain are entirely marine, and are chiefly lamellibranchs and gastropods—for example, Corbula regulbiensis and Aporrhais Sowerbyi.
The Woolwich and Reading Beds are more variable. In the east of Kent the series consists throughout of light-coloured false-bedded sands containing marine fossils. In the western part of Kent and in East Surrey it is formed partly of sands, partly of grey clay, generally full of estuarine shells, sometimes with bands of oysters. Towards Reading it consists of mottled plastic clay and variegated sands, generally unfossiliferous, but sometimes with remains of plants. The Beading type occurs also along the northern margin of the basin, in Hertfordshire and Essex. Sometimes it includes a bed of pebbles cemented into a hard conglomerate known as the Hertfordshire pudding-stone.
Evidently during the deposition of this series the sea lay towards the east,the land towards the west. An estuary lay over East Surrey and West Kent, and probably the plant-bearing plastic clays are the freshwater deposits laid down by the rivers which flowed into the estuary.
Amongst the marine fossils of this series are Ostrea bellovacma, Cyprina Morrisi, &c.; amongst the fresh-water and estuarine forms are Unio, Cyrena, Viviparus. Remains of fishes, reptiles, birds and mammals have also been found.
In the neighbourhood of London the sands of the Woolwich series are overlaid by a considerable thickness of current-bedded gravels which have sometimes been distinguished as a special subdivision under the name of the Blackheath Pebble-Beds. They consist chiefly of well- rounded flint-pebbles in a fine sandy matrix. The junction with the sands below is usually sharp and often very irregular, as if the surface of the sands had been eroded.
In the eastern part of Kent the place of these pebble-beds is taken by the Oldhaven Beds, which consist of fine drab-coloured sand, with a bed of flint- pebbles at the base.
Palaeontologically neither the Blackheath nor the Oldhaven beds have any distinctive characters. Generally the fossils which they contain are much the same as those of the beds below, but sometimes they approach more nearly to the London Clay. No sign of these deposits has been met with in the west or north of the London Basin, and they were evidently a local bank of shingle on the floor of the Eocene sea.
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