THE STORY OF THE ISLAND RIVERS; AND HOW THE ISLE OF WIGHT BECAME AN ISLAND
We must now consider the history of the river system of the Isle of Wight, to which our study of the gravels has brought us. For rivers have a history, sometimes a most interesting one, which carries us back far into the past. Even the little rivers of the Isle of Wight may be truly called ancient rivers. For though recent in comparison with the ages of geological time, they are of a vast antiquity compared with the historical periods of human history.
To understand our river systems we must go back to the time when strata formed by deposit of sediment in the sea were upheaved above the sea level. To take the simplest case, that of a single anticlinal axis fading off gradually at each end, we shall have a sort of turtle back of land emerged from the sea, as in [figure 6, aa] being the anticlinal axis. From this ridge streams will run down on either side in the direction of the dip, their course being determined by some minor folds of the strata, or difference of hardness in the surface, or cracks formed during elevation. On each side of the dip-streams smaller ones will flow, more or less in the direction of the strike, and run into the main streams. Various irregularities, such as started the flow of the streams, will favour one or another. Consider three streams, a, b, c, and let us suppose the middle one the strongest, with greatest flow of water, and cutting down its bed most rapidly. Its side streams will become steeper and have more erosive force, and so will eat back their courses most rapidly until they strike the line of the streams on either side. Their steeper channels will then offer the best way for the upper waters of the streams they have cut to reach the sea; and these streams will consequently be tapped, and their head waters cut off to flow to the channel of the centre stream. We shall thus have for a second stage in the history a system such as is shown in [fig. 7]. The same process will continue till one river has tapped several others; and there will result the usual figure of a river and its tributaries, to which we are accustomed on our maps. We shall observe that tributaries do not as a rule gradually approach the central stream, but suddenly turn off at nearly a right angle from the direction in which they are flowing, and, after a longer or shorter course, join at another sharp angle a river flowing more or less parallel to their original direction.
Fig. 6
Fig. 7
Development Of River Systems
The Chalk and overlying Tertiary strata were uplifted from the sea in great folds forming a series of such turtle-backs as we have been considering. The line of upheaval was not south-west and north-east, as that which raised the older formations in bands across England, but took place in an east and west direction. The main upheaval was that of the great Wealden anticline. Other folds produced the Sandown and Brook anticlines, and that of the Portsdown Hills. The upheaval seemed to have been caused by pressure acting from the south, for the steeper slope of each fold is on the northern side. Our latest Oligocene strata are tilted with the chalk, showing that the upheaval took place after Oligocene times. But the great movement was in the main earlier than the Pliocene. For on the North Downs near Lenham is a patch of Lower Pliocene deposit resting directly on the Chalk, the older Tertiary strata having been removed by denudation, clearly due to the uplift of the Wealden anticline. The raising of the Pliocene deposit to its present position proves that the same movement was continued at a later time, probably during the Pleistocene. But the greater part of the movement may be assigned to the Miocene, the period of great world-movements which raised the Alps and the Himalaya.