C. E. de Ranee — Pre- Glacial Geography of N. Cheshire. 159 



being the landward edge of this plain. It is nowhere better seen 

 than in the Birket plain, forming the northern portion of the Hun- 

 dred of Wirral, in North Cheshire. It is bounded to the south by 

 an old pre-Glacial cliff, which abruptly terminates the northerly pro- 

 longation of all the numerous longitudinal valleys running with the 

 strike of the Triassic rocks, of which this district is composed, 

 each valley having a steep escarpment facing the west, as described 

 by Professor HulP and myself." To the east the continuity of this 

 plain with the Mersey and with that surrounding Lancashire further 

 to the north, is broken by a comparatively high tract of Triassic 

 sandstones, rising like an island between the Mersey and the plain ; 

 to the north this tract is bounded by the sea, which is forming cliffs, 

 and to the south its continuity is broken by a transverse gorge run- 

 ning across the strike of the strata, at the bottom of which flows to 

 the east the tributary of the Mersey called Wallasey Pool. The 

 western margin of the tract is above the plain, consisting of a 

 natural escarpment, a continuation of that forming the eastern slope 

 of a valley south of the Pool. On the side of the Mersey the eastern 

 face of the tract is concealed by a bed of Glacial Drift, nearly 

 seventy feet in thickness, forming a cliff of about that height ; these 

 deposits also fill up the bottom of Wallasey Pool gorge, and cover to a 

 great extent the undulating plains stretching away from Liverpool 

 towards Ormskirk, on the opposite side of the Mersey. The banks 

 of the latter, on the north side, are sometimes composed of about 

 twenty feet of rock, capped by Boulder-clay, but more often the 

 latter comes down to high-water mark, and occasionally even far 

 below, proving that the deepest line of the pre-Glacial valley did not 

 precisely correspond in position to the present channel of the river. 

 This, taken in conjunction with the fact that the bottom of the 

 Wallasey gorge, as' well as the rock floor of the Birket plain, is ex- 

 cavated far below high- water mark, the space being filled in with 

 Boulder-clay, leaves little doubt that in pre-Glacial times the land 

 stood at least thirty feet higher than at present. The breadth of the 

 actual stream of the Mersey is inconsiderable compared with the 

 present extent of its estuary, which is produced by the bottom of the 

 old valley being submerged beneath the tidal level, but not until 

 the Boulder Drifts had been re-excavated out by post-Glacial de- 

 nudation, which, in the case of the Mersey, was chiefly fluviatile, 

 but in the Birket plain was marine, the sea wearing back the Boulder- 

 clay up to the old pre-Glacial east and west cliff, and also slightly 

 cliffing the base of the natural escarpment, forming the western 

 margin of what may be called the " Wallasey island." This Wal- 

 lasey escarpment exhibits fine sections of the unconfoi-mable junction 

 of the Keuper with the upper mottled sandstone, and was once con- 

 tinuous with that of Flay brick Hill. These tv/o form the eastern 

 slope of an old longitudinal valley before the formation of the gorge, 

 the western slope of which has been removed by that denudation 

 which produced the Birket plain, and also destroyed the seaward 

 prolongations of all the existing north and south valleys further to 

 1 Explanation of Hor. Sec, Sheet 68- 2 Qy^r. Journ. Geol. Soc, Feb. 1871. 



