Vol. 68.] POST-GLACIAL CHAISTGES IN THE LOWER DEE VALLEY. 197 



IV. Conclusion. 



Owing to obstruction of the peculiarly serpentine course of the 

 whole valley of the Dee near Llangollen by ice and Glacial deposits, 

 overflow channels across the necks of several loops were initiated. 

 Two of these have become permanent on the retreat of the ice and 

 are now river-gorges, while the loops are left as dry valleys. 



Near Cefn the river enters a long post-Glacial gorge, which 

 extends to beyond Overton Bridge. The pre-Glacial valley of the 

 .Dee is traceable beyond Chirk in a south-easterly direction to near 

 St. Martin's Moor. 



The Drift-filled valley, shown by Dr. Strahan to exist below 

 the estuary of the Dee and as far south as Pulford, extends to 

 Kodens Hall near Bangor-on-Dee, M^here it is still 30 feet below 

 sea-level. This indicates a much steeper pre-Glacial ' thalweg ' 

 than the present one. 



An attempt is made to prove that these two valleys are 

 continuous, although the intermediate portion is obliterated by 

 Drift. 



The question is raised, whether the uplift during which the 

 erosion of this deep buried valley took place was pre-Glacial, or 

 whether it occurred during the Glacial Period. The evidence is 

 far from conclusive, but appears to point to the latter view. 



EXPLANATION OF PLATE XI. 



Map of part of the Kiver Dee, to show the distribution of the Drift and 

 the pre-Glacial topography, on the scale of 2 inches to the mile, or 

 1 : 31,680. 



Discussion. 



Mr. G. W. Lamplugh said that, having seen the field-evidence 

 with the Author, he could fully support the data given in the paper. 

 The gap between the buried valley near Chirk and that under the 

 estuary of the Dee might be bridged by deep borings in the future ; 

 but there was always the possibility of surprises in the hidden 

 channels, as the drainage-system of the whole of the lowland 

 country had been profoundly modified by the Glacial episode. All 

 over the glaciated parts of the British Islands it was common to 

 find that the rivers, on leaving their pre-Glacial valleys of the hilly 

 country, had gone widely astray, and the Author had now brought 

 the Dee into this class. 



Dr. A. Wade referred to a recent paper, read before the Liverpool 

 Geological Society, in which he had produced evidence to show that 

 the Welsh Border valleys immediately to the south of the Dee 

 Valley had been occupied by Glacial lakes. He asked the Author 

 whether he had considered an explanation of this kind for some of 

 the phenomena which he had described. 



Dr. H. Lapworth called the Author's attention to the buried 

 pre-Glacial river-course below Boulder Clay in the valley of the 



