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Show 284 EOCENE PERIOD. [Ch. XX, those spaces where the outliers above mentioned occur, nor that the comparative thinness of those deposits in the higher chalk countries should be attributed chiefly to the greater degree of denudation which they have there suffered. Origin of the English tertiary strata.-In explanation of the phenomena above described, we shall endeavour, in the two next chapters, to lay before the reader a view of the series of events which may have produced the leading geological and · geographical features of the south-east of England. A preliminary outline of these views may be useful in this place. We conceive that the chalk, together with many subjacent rocks, may have remained undisturbed and in horizontal stratification until after the commencement of the Eocene period. When at length the chalk was upheaved and exposed to the action of the waves and currents, it was rent and shattered, so that the subjacent secondary strata were exposed at the same time to denudation. The waste of these rocks, composed chiefly of sandstone and clay, supplied materials for the tertiary sands and clays, while the chalk was the source of flinty shingle, and of the calcareous matter which we find intermixed with the Eocene clays. The tracts now separating the basins of London and Hampshire were those first elevated, and which contributed by their gradual decay to the production of the newer strata. These last were accumulated in deep submarine hollows, formed probably by the subsidence of certain parts of the chalk, which sank while the adjoining tracts were rising. CHAPTER XXI. Denudat~on of secondary strata during the deposition of the English Eocene formabous-Valluy of the Weald between the North and South Downs-Map -Secondary rocks of ,the Weald divisible into five groups-North and South Downs-Section across the valley of the Weald-Anticlinal axis-True scale of heights-Rise and denudation of the strata gradual-Chalk escarpments once sea-cliffs-Lower terrace of 'firestone,' how caused-Parallel ridges and valleys formed by harder and softer beds-No ruins of the chalk on the central district of the Weald-Explanation of this phenomenon-Double system of valleys, the longitudinal and the transverse-Transverse how formed-Gorges intersecting the chalk-Lewes Coomb-Transverse valley of the Adur. Denudation of the Valley of the Weald.-IN order to understand the theory of which we sketched an outline at the close of the last chapter, it will be necessary that the reader should be acquainted with the phenomena of denudation exhibited by the chalk and some of the older secondary rocks in parts of England most nearly contiguous to the basins of London and Hampshire. It will be sufficient to consider one of the denuded districts, as the appearances observable in others are strictly analogous; we shal1, therefore, direct our attention to what we may call the Valley ofthe Weald, or the region intervening between the North and South Downs. Map.-In the coloured map given in Plate V. *,the district alluded to is delineated, and it will be there seen that the southern portion of the basin of London, and the north-eastern limits of that of Hampshire, are separated by a tract of secondary rocks, between 40 and 50 miles in breadth comprising within it the whole of Sussex and parts of the c~unties of Kent, Surrey, and Hampshire. There can be no doubt that the tertiary deposits of the Hampshire basin formerly extended much farther alonO' our southern coast towards Beachy Head, for patches ar: still * This map has been chiefly taken from Mr. Greenough's Map of England. |
