182 MR. L. J. WILLS OTST LATE GLACIAL AND [Juiie I912, 



and gravel high up on the Millstone Grit outcrop near Pron b}' 

 Craignant, where rolled pellets of red boulder-clay with scratched 

 boulders in it occur as pebbles. 



[b) The Welsh Drift. 



The Welsh Drift, both Boulder Clay and gravel, is essentially 

 made up of Silurian and Ordovician rocks. The clay is a very 

 stony, grey till, often containing 80 or 90 per cent, of stones, 

 the majority beiug flat fragments of Silurian shale. The gravels 

 are practically the same material, slightly washed and rearranged ; 

 and the two deposits are not readily distinguished, even in good 

 exposures. 



As regards the distribution, it may be said that, wherever in the 

 neighbourhood of Chirk the Irish Sea and Welsh Drifts are associ- 

 ated, the Welsh is the later formation. 



North of the Dee Valley, Welsh Drift of a typical composition is 

 not well developed east of the Millstone Grit Escarpment ; but it 

 is the only Glacial deposit seen at low levels in the Dee ^''alley 

 above Pron Cysyllte,^ and has been found near that locality under 

 the recent alluvium. 



The ground between the Dee and the Ceiriog, and west of the 

 Black Park — Chirk Green ridge, is chiefly occupied by Welsh Drift, 

 and it is seen overlying red drift up to Castle Mills in the Ceiriog 

 valley. In the upper part of this valley it is the only Glacial 

 deposit found. 



South of the River Ceiriog the dip-slopes of the Millstone Grit 

 are covered by a series of rude terraces composed of the same Welsh 

 Drift, although there is evidence at one or two places here that it 

 overlies red Irish Sea Boulder Clay. 



The Welsh gravels — best described as ' slaty gravel ' — are chiefly 

 found in the Dee Valley as rough, high-level terraces. Similar 

 deposits spread out as flats at Chirk and Brynkinalt, and in the 

 Ehyn Park. In these cases it is clear that they are flood-gravels, 

 belonging to the early post-Glacial Ceiriog drainage. 



JSTear Chirk Bank the gravels form esker-like ridges and mounds, 

 and are less monotonously Silurian in origin. 



The difficulty of distinguishing Welsh Boulder Clay from the 

 ' slaty gravel ' is increased in the Ceiriog valley, by the fact that 

 some of the Boulder Clay assumes a terrace-like form. 



III. Post-Glacial Changes in the Topogeapht of the Disteict. 



It may be well to preface this discussion by a definition of post- 

 Glacial changes as those which have been effected since the time of 

 the maximum giaciation. They were probably initiated at a late 

 stage in the Glacial Period. 



In pre-Glacial times there existed in the district the same two 

 chief drainage-lines — the Dee and the Ceiriog — as at the present 



1 Misspelt ' Cysysllte ' on the map, PL XL 



