A HISTORY OF KENT ments of the earth's crust during Miocene times by which huge mountain chains were upheaved in some parts and vast sheets of molten lava poured out over the surface in others. In a minor but still important degree these disturbances affected the whole of the south-east of England, throwing the rocks into broad waves, or buckling them into sharp folds such as may be seen in the cliff-sections of the Isle of Wight and of Dorset. Although the already-described ' overstep ' of the upper beyond the lower divisions of the Lower London Tertiaries indicates that the eleva- tion of the Wealden dome must have begun very early in Eocene times, it was probably under the influence of these great earth-movements of the Miocene period that the principal uplift took place. And as we shall presently see, the form that was then given to the surface is still reflected in the river-systems of the county, which must have been established when the outline of the land was very different from that which it now presents. It was after this elevation that the chiselling of the surface commenced of which the existing relief is the distant outcome. Once however in the interval between the Miocene uplift and the present time the area must have been temporarily submerged beneath the sea, as the following evidence will show. PLIOCENE PERIOD Lenham Beds. — Along the crest of the Downs from the coast above Folkestone to within a few miles of Maidstone, the Chalk is capped here and there with patches of rusty sand sometimes indurated into lumps of ironstone. This material is usually unfossiliferous, but in two or three places the hollow casts of marine shells have been found in the ironstone, and these are sufficient to indicate that the deposit is of Older Pliocene age, equivalent to the Diestian Beds of Belgium and to the lower part of the Coralline Crag of Suffolk.' The sands appear originally to have been glauconitic and full of shells, but have been slowly weathered into their present condition by the percolation of surface-water through them ; so that were it not for the preservation of the casts in the iron- stone, from which it is possible to obtain determinable moulds of the shells,* they would have been devoid of direct evidence as to their age. The principal locality for these fossils is at Lenham, nine miles east of Maidstone, where the sands and ironstone have sunk down into ' pipes ' or deep cylindrical holes melted out in the Chalk by the solvent action of the surface drainage in passing along ' water-sinks.' The fauna, which is exclusively marine, comprises species of Turritella, Pyrula, Pectunculus, Area, Terebratula, etc., and is believed to indicate a depth of the sea of not less than 40 fathoms during the accumulation of the sands. ^ 1 Mr. F. W. Harmer has recently expressed the opinion that the Lenham Beds are slightly older than the Coralline Crag. See Quart. Joum. Geol. Sec. (1900), Ivi. 708. 2 See C. Reid, Nature (1886), xxxiv. 341. 3 Mem. Geol. Survey, 'Pliocene Deposits of Britain,' p. 52.
