Page:VCH Sussex 1.djvu/59

 GEOLOGY surface of Chalk, all the intermediate strata having been tilted up and denuded before the Pliocene were deposited : therefore the disturbance had taken place before the Pliocene period. This narrows the limit of time during which the great folding occurred to some part of the Miocene, or perhaps of the preceding Upper Oligocene. More direct evidence obtainable on the continent shows that the Miocene was one of the great periods of earth movement and mountain building, and to this period we may therefore safely refer most of the folding in Sussex. The movement in Sussex seems to have consisted of a horizontal compression of the strata from north to south, by which they were bent into a series of folds having an east and west axis. Thus was formed the large anti- clinal arch of the Weald and the syncline of the Hampshire basin, as well as the numerous smaller ripples which will be found indicated on the geological map. To the same period belong the very curious over- thrust faults so well seen on the foreshore between Eastbourne and Beachy Head, though these happen to run north and south for a short distance, for they apparently occur just where one fold is dying out and a fresh one commencing. All the folds are elongated domes, arranged en echelon, not in continuous ridges ; where one fold dies out a new one commences, but not exactly in the same line and not continuous with it. The lateral compression of the rocks just referred to necessarily caused them to expand upwards, in the only direction in which they were free, to form east and west ridges. The largest of these undulations would now form a mountain chain over 6,000 feet in height in the centre of the Weald, were it not that rivers and sea combined to plane it down almost as fast as it rose. Its uprise however was sufficiently rapid to determine the course of the Wealden rivers, which flowed down the northward and southward slopes, diverging from the Wealden axis. During subsequent periods the country around this axis, being formed of rocks more easily denuded than the Chalk, has become lowered much below the level of the Downs through which the rivers now flow in narrow and deep valleys. A river once started tends to deepen its channel, but remains nearly in the same place long after the original slopes which first directed its course have been obliterated by the erosive action of its tributaries. The high cliff-like escarpment of the South Downs, which overlooks the Weald, is due to the erosive power of rain and rivers acting on strata some of which are hard and some soft ; it is not due to the waves of the sea as formerly thought. Standing on the Downs and overlooking the low-lying plain it is difficult to believe that we are not looking across the bed of an ancient sea, which once filled the Weald. But not only are newer Tertiary marine deposits absent from the Weald, but as Mr. Whitaker has pointed out, escarpments can readily be distinguished from sea-cliffs by certain characteristics. The foot of a sea-cliff keeps to one level, but cuts through various strata ; the foot of an escarpment formed by rain and rivers rises and falls consider- ably, but keeps to the same geological horizon. The northward-facing slope of the South Downs is an escarpment always having at its base the I 17 3