378 MK. A. STEAHAN ON THE GrLACIATION OF 



drift thins out against an abrupt bank, forming the edge of a coarse 

 gravel plateau of Welsh origin. As a general rule it may be stated 

 that where there exists a strong physical feature between the hills 

 and the plains, this feature, whether formed by drift or solid rock, 

 will also form the limit of the northern drift. Where, on the other 

 hand, the change of level is gradual, the limit of the northern drift 

 runs to greater altitudes, and encroaches on the Principality. 



The direction in which the drifts have been transported in the 

 Welsh borders is in many cases easily ascertainable. The Coal- 

 measures, Millstone Grit, Carboniferous Limestone, and Wenlock 

 Shale of Flintshire form four roughly parallel belts of rocks of 

 sufficiently marked lithological character to enable their detritus to 

 be traced. In each case the drift-material has been transported 

 approximately from south-west to north-east. The margin of the 

 coal-field is overspread by debris from the Lower Carboniferous 

 rocks ; the Millstone-Grit area, as for example near Treiddyn, is 

 thickly covered by a Boulder- clay crammed with great boulders of 

 sandstone and limestone ; while the limestone, especially in North 

 [Flintshire, near Caerwys and Cwm, is concealed beneath a yellow 

 clay, packed with fragments of W^enlock Shale from the neighbouring 

 Moel Fammau range. The distribution of red drift derived from the 

 Bunter Sandstone of the Yale of Clwyd furnishes equally strong 

 evidence. The bold and almost continuous barrier formed by the 

 Moel Fammau range on the east side of the Vale of Clwyd is 

 breached at Bodfari by the valley of the Wheeler, which is 

 continued between the Silurian rocks and the Flintshire limestone 

 escarpment by Caerwys, Nannerch, and Mold, until it becomes the 

 valley of the Alyn, the valley itself, though occupied by different 

 rivers in its different parts, forming a continuous breach through 

 the successive hill-ranges. Up this valley the red debris of the 

 Triassic rocks of the Yale of Clwyd has been transported in such 

 masses, as to have led to the belief that the Trias itself ran up 

 between the Wenlock-Shale hills. On both sides of the Wheeler, 

 above Bodfari, the bright red sand mantles up the hill-sides or rises 

 into eskers and mounds ; and though the red tinge grows gradually 

 fainter, on receding from the Yale of Clwyd, it is not till the 

 neighbourhood of Mold is reached that it is finally lost. The 

 sections in the banks of the river near Caerwys and Nanuerch show 

 that this red sand is interstratified with a gravelly drift composed 

 of fragments of AYenlock Shale from the Silurian hills. It should 

 be noted, however, that a red drift-sand has been observed by 

 Prof. Hughes in the gorge of the Elwy as far up as Dol *, which, if 

 correctly identified as Clwydian drift, seems to have travelled in a 

 westerly direction. It is possible that this part of the gorge came 

 within the infiuence of the northerly currents by which drifts of 

 northern derivation were spread over the lower part of the Yale of 

 Clwyd. 



The same story of easterly drift is told by the larger boulders 



■■' Proc. Chester fcoe. Nat. Sci. part 3 (1885). 



