480 ME. E. W. HAEMEE ON THE [Nov. I907, 



dammed lakes, occur in many places on the lower ground, though 

 they may be traced also to a height of about 500 feet O.D. 1 



They have described, moreover, terraces of sand and gravel 

 which they regard as the marginal deposits of a Glacial lake or 

 lakes, the different levels at which they occur representing successive 

 stages in the retreat of the water. 2 Lacustrine conditions may have 

 also obtained in this district, they consider, during the advance of 

 the ice. 



Farther south, towards Shrewsbury, during a visit in the autumn 

 of 1906, Mr. E. H. Pastall and I found similar laminated brick- 

 earths to the north-west of Ironbridge, at Great Hanwood, Uffing- 

 ton, Astley, Shawbury, and Leaton. 



In the present instance, as in others here dealt with, the deepest 

 part of the supposed lake was near the trumpet-shaped entrance of 

 the gorge through which it was drained. 3 Such features may have 

 been due to the swirl of the water, converging to one point, where 

 its erosive power was greatest. 



Conditions like those existing at Ironbridge, namely, an isolated 

 area of depression, drained by a straight and narrow channel, are 

 reproduced on a smaller scale near the junction of the Vyrnwy and 

 the Severn, the excavation of the lakelet at that point having also 

 taken place pari passu with the deepening of its outlet (see 

 PI. XXXI). 



As in the Vale of Pickering, all traces of the former drainage- 

 system of this region have disappeared. The Yyrnwy and the 

 Severn must have originally excavated valleys in the Triassic 

 deposits of the Cheshire plain ; there are borings in the latter 

 which show the sub- Glacial contours to be in places very irregular. 

 At Sydney, for example, near Crewe, on low ground, the Glacial 

 Drift was found to be 320 feet thick and to descend 160 feet below 

 the present sea-level. At a short distance from the same place 

 the sub-Glacial surface of the Keuper Marl rises at least 200 feet. 4 



It would appear, therefore, that the levelling of the Cheshire 

 plain was not due to fluviatile action, cutting it down to base-level, 

 but to the movement of the ice-sheet over it, on the one hand, 

 and the accumulation of Glacial Drift, lacustrine sediment, or 

 alluvium on the other. In other parts of the glaciated area ancient 

 valley-systems have been concealed by Glacial and post-Glacial 

 deposition, rather than obliterated by the action of rivers. In some 

 localities, however, it is clear that erosion has helped to produce 

 the areas of depression of which this paper treats, and especially 

 the deeper part, the lake within a lake (as shown on a contour-map), 

 which is found near the mouths of the overflow-gorges. 



1 Briekearth of a somewhat different character, passing laterally into ground- 

 moraine, rises in places along the flanks of the Pennines to a height of 600 feet, 

 ' The Geology of the County around Macclesfield, &c.' Mem. Greol. Surv. 1906, 

 p. 89. 



2 Ibid. p. 93. 



2 Trumpet-shaped, that is, horizontally, as shown in a contour-map. 

 4 Op. supra cit. p. 68. 



