196 MK. L. J. WILLS ON LATE GLACIAL AND [June I912, 



past Hordley and EUesmere to Overton, and so along a line west of 

 l3angor-on-I)ee, where a small patch of Trias emerges from the 

 Boulder Cla}-, to Eodens Hall. This course is hj'^pothetical, and 

 has yet to be proved by borings. 



(9) Points of Theoretical Interest. 



There are one or two points of theoretical interest which are 

 suggested by the foregoing observations. 



I have assumed throughout the discussion that the work of 

 Messrs. Lamplugh, Tiddeman, Wright/ and Maufe on the pre-Glacial 

 beaches round our coasts is sufficient to prove that the pre-Glacial 

 sea-level, south of the T^'ne, was approximately the same as the 

 present one. This removes the possibility of differential earth- 

 movement having occurred within the area under discussion. 



Dr. Strahan's observations have indicated an uplift of at least 

 184 feet at the mouth of the Dee. The varying depths at which 

 the pre-Glacial surface lies beneath the Drift near the mouths of 

 most of our northern rivers suggest either Glacial deepening, or 

 that the uplift was considerably greater than 184 feet. In the 

 latter case, the discrepancies could be explained by the fact that 

 these areas were not then situated near enough to the sea for the 

 valleys to be graded to the base-level of the sea at these points. 



The youthfulness of the Dee Valley in the hill country is among 

 its most striking features, and accords well with a hypothesis of 

 recent uplift. My observations on the slope of the 'thalweg' of 

 the pre-Glacial Dee also indicate that the valley was a ' young ' one, 

 and suggest that the uplift was sudden and not of long duration. 

 Too much reliance cannot be placed upon this evidence, on account 

 of the possibility of the sub-Glacial surface being the result of 

 glacial erosion, to which the soft Triassic rocks would be very 

 susceptible. 



If we accept the hj-pothesis of an uplift, did it take place before 

 or after the formation o£ the pre-Glacial beaches ? If it was before, 

 it follows that a re-subsidence had occurred to the present level 

 before glaciation, and in this case the Drift filling in the Dee Yalley 

 as far as Kodens Hall, and probably much farther south, must have 

 been deposited in an arm of the sea. There is no satisfactory 

 evidence of this, however, in the composition of the Drift. 



If, on the other hand, both elevation and re-subsidence occurred 

 after the formation of the beaches, they are episodes in the com- 

 paratively short Glacial and post-Glacial Periods. The steepness of 

 the thalweg and the strength of the eroding forces would fit in 

 satisfactorily with this view. 



^ For references, see W. B. Wright, ' On a Pre-Glacial Shore-Line in the 

 W^estern Isles of Scotland ' Geol. Mag. dec. 5, vol. \iii (1911) pp. 97-109, 





