160 C. E. de Ranee — Pre-Glacial Geography of N. Cheshire. 



tlie westward towards the Dee, now abruptly cut short hy the cliff 

 forming the southern margin of the Birket plain and Wallasey Gorge. 

 Before the formation of the former plain/ and before the north and 

 south valleys were deepened to their present extent, it is clear that 

 the Mersey flowed westward over a plain, the level of which coin- 

 cided with, or was but little above, that of the various escarpments 

 occurring between the valleys. For when it first commenced to flow 

 they were not in existence, and the streams which afterwards flowed 

 in them could only deepen them in regard to the gradually decreas- 

 ing level of the transverse gorge, which regulated the height of the 

 outfall. Therefore, it is not improbable that when the river 

 flowed over this old plain, that it extended far to the westward, until 

 it abutted against the mountains of Wales, and that the hard and 

 soft beds composing it were alike level — longitudinal valleys not 

 being yet scooped out of the latter. For in all districts where these 

 phenomena are observable the slope of the soft beds composing a 

 longitudinal valley is towards the brook or stream, which runs at 

 right angles to an escarpment, forming the landward side of it, and 

 which, in cutting through escarpments lower down, forms trans- 

 verse gorges, the upper termination of the slope corresponding to 

 the level of the base of the escarpment. It is therefore clear that 

 before the latter was formed these longitudinal slopes could have had 

 no existence, and that their angle increased exactly in proportion 

 to the height gained by the escarpment, or in other words to the 

 descent of the level of the base. Therefore, there is reason to believe, 

 that the various valleys south of the old high-level course of the 

 Mersey did not extend indefinitely northwards, but fell into that river 

 at points, opposite which entered similar streams, flowing from the 

 north. The continuity of the old plain westwards was broken by the 

 Eiver Dee, which must have formed a broad v^ley, receiving the 

 waters of the Mersey as a tributary, at a point opposite Mostyn ; 

 from thence, flowing northwards, far out into what is now the Irish 

 Sea, and receiving the waters of the Clwyd. As the Mersey deepened 

 its course, longitudinal valleys began to be formed by the wash of 

 rain on the soft sandstone, and after a time the hard beds began to be 

 left as escarpments, through which the river always cut a channel 

 hefore the soft beds behind were removed by lateral wash, which could 

 never work lower than the level of the outfall, the key to which was 

 the transverse stream. Subsidence taking place, marine denudation 

 went on, and the sea gradually approached the district, the valleys 

 and hills to the west were shaved across, and the Birket low-level 

 plain came into existence, at a level but slighly below that to which 

 the longitudinal valleys had worn themselves ; the Mersey flowed at 

 the bottom of the Wallasey Pool gorge, and fell directly into the sea. 

 In historic times the Wallasey Pool was a main outlet of the 

 Mersey, but at present it enters the sea by a broad valley between 

 Liverpool and Egremont ; but it would appear probable, as this is a 

 longitudinal valley, with a fault at the bottom, lying north of the 

 Wallasay Gorge, that in pre-Glacial times the stream that occupied it 

 flowed south into the Mersey, and that ou the deepening of the 



