Page:VCH Kent 1.djvu/70

 A HISTORY OF KENT In discussing this branch of our subject it is important to remember that there are no deposits in Kent that can be directly assigned to the glacial agencies which produced such widespread effects in the country north of the Thames. The great ice-sheet that gradually crept over all the northern lands of Europe after the close of Pliocene times seems to have attained its southerly bounds at the estuary of the Thames, so that Kent lay just beyond its margin. But during this Glacial Period the conditions must have been even more favourable to unequal or valley erosion in the bare country, exposed to alternate freezing and thawing and to heavy floods derived from the melting of the winter snows, than in the county farther north, which was to some extent protected by its mantle of permanent ice. Hence, while boulder-clays and glacial gravels were being outspread upon the land to the northward, torrential denudation was rapidly cutting into the Kentish hills and sending turbid floods, active in erosion, along its main valleys. There has been much dis- cussion as to the exact relationship between the Glacial drifts of the north of England and the fossiliferous gravels and brickearths of the Thames valley,* the circumstances being of course unfavourable for direct correlation. Some part of the older ' superficial ' deposits of Kent are likely to be at least of Glacial age, but as subaerial conditions were persistent throughout the period and have continued to prevail up to the present time, the fragmentary evidence which remains is scarcely sufficient to enable us to recognize the limits of the period in Kent. Recent Deposits. — Between the deposition of the old river-drift with remains of extinct mammals and present-day conditions there have been many intermediate stages, of which some record is pre- served in the lower terraces and recent alluvium of the valleys. These newer deposits all indicate a shrinkage in volume of the rivers, and they also show that within comparatively recent times the land has stood somewhat higher than at present. Excavations for docks and other works below the level of high tide in the Thames valley below London, especially between Woolwich and Erith, have revealed layers of peat with trunks of trees, including the oak and yew, indicating forest growth in situ, this peat being interstratified with beds of marsh clay, the whole resting on river-gravel and sand. Where fossils occur in these deposits they are all of species still living ; and traces of human work of Neolithic and later date are also occasionally found. The marshes of the lower Thames and of the mouths of the Medway and the Stour are further examples of these recent alluvia, and less extensive deposits of the same kind fringe the streams in the interior. are West Wickham, Swanscombe, Milton Street, Ash, Darent, Rainham, etc. G. Clinch, ' On Drift Gravels at West Wickham,' Quart. Jouitt. Geo!. Soc. (1900), Ivi. 8 ; J. M. Mello, 'On some Palaeolithic Implements of North Kent,' Re/i. British Assoc, for 1899 (Dover), p. 753, etc. See also article on ' Early Man ' in present volume. 1 For summary, see Mem. Geol. Survey, 'Geology of London,' pp. 353-87. 24