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 A HISTORY OF SUSSEX soft Lower Chalk ; the southward-facing bluff (still to be described) is a true sea-cliff, which sometimes leaves the Chalk altogether and cuts through Tertiary strata. The Sussex rivers and their peculiar courses will best be understood from an examination of the accompanying orographic map. It will be noticed that the principal streams, Arun, Adur, Ouse and Cuckmere, rise on comparatively low ground towards the centre of the Weald and make a short cut to the sea through gaps in the South Downs. The Ashburn and the Rother, on the other hand, now flow over low country to fall direct into the English Channel ; but it is possible that they also at one time behaved like the other Wealden rivers and breached the Chalk hills at a time when the Downs extended more to the east, Topley, who did so much to elucidate the whole question of the origin of the Wealden rivers, thought that formerly the Ashburn, which rises on the south side of the axis, broke the South Downs a few miles east of Beachy Head, and that the Rother, which rises north of the axis, turned northward and breached the North Downs somewhere near the middle of the present Straits of Dover. ^ It seems doubtful however whether within the lifetime of the existing rivers the South Downs were ever continuous with the Chalk hills of France,^ though the North Downs appear to have been so, for chalk has been traced across the bed of the strait from shore to shore. When we try to fix a date for the beginning of this peculiar valley system it is obviously needless to look back to times anterior to the last period when the county was submerged beneath the sea ; for the sea tends to plane down the hills and to level up depressions, so that any previously existing valleys are not likely to reappear when dry land again emerges. The latest submergence to any considerable depth seems to have been of older Pliocene date, marine deposits of this period capping the North Downs at a height of over 600 feet near Lenham, though they have not yet been discovered in Sussex. It does not seem probable that any of the existing valleys date from an earlier period, though there may have been an older system having a similar relation to the Wealden uprise. It is not easy to follow the exact course of events in later Plio- cene times, when the land rose and the streams began their work ; for the rivers have long since entirely destroyed their earlier deposits, so that no fossil relic now remains in Sussex of the interesting fauna and flora which overspread Britain in preglacial times. From records preserved in other counties we learn that these were times when large animals abounded — elephants, rhinoceroses, hippopotami, numerous deer, mostly of species unknown in later deposits, besides animals of more unfamiliar type, such as the sabre-toothed tiger and mastodon. Towards the close of the Plio- cene period Britain was still joined to the continent, the Thames and the rivers north of the Wealden axis being tributaries of a larger Rhine, which then seems to have reached the sea somewhere off the present Norfolk ' ' Geology of the Weald,' chap. xvi. and plate ii. '^ Reid, 'Geology of Eastbourne,' Mem. Geo/. Survey (1898), p. 13. 18