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. Many remarkable, and, at first sight, very puzzling features connected with the courses of rivers find an explanation when we study the river history. Thus, looking at the Weald of Kent and Sussex, we see that it consists of comparatively low ground rising to a line of heights east and west along the centre, and surrounded on all sides but the south-east by a wall of Chalk downs. If we considered the subject, we should suppose that the drainage of the country would be towards the south-east, which is open to the sea. Not so. All the rivers flow from the central heights north and south,--go straight for the walls of chalk downs, and cut through the escarpment in deep clefts to flow into the Thames and the Channel. This is explained when we remember that the rivers began to flow when the great curve of strata rose above the sea. Though eroded by the sea during its elevation, yet when it rose above the waters the arch of chalk must have been continuous from what are now North Downs to South. And from the centre line of the great turtle back the streams began to flow north and south, cutting in the course of ages deep channels for themselves. The greater erosion in their higher courses has cut away the mass of chalk from the centre of the Weald, but the rivers still flow in the direction determined when the arch was still entire.