44 THE DENUDATIONS OF NORTH WALES. 
What was the condition of North Wales through all the 
periods of the Lias, the Oolites, the Chalk, and the Tertiary 
beds, there is very little evidence to shew. But inasmuch as the 
Chalk may be seen in other parts to overlap all the lower 
Secondary formations towards the West, it may be assumed that 
the submergence during the Chalk was more general than in the 
preceding periods; it is not improbable even that this formation 
may have covered a portion, if not the whole, of North Wales, 
and have joined up those isolated fragments which still exist in 
the North of Ireland under a protective covering of Miocene 
basalt. It must be remembered that if neither the Chalk nor 
any of the earlier Secondary formations overspread North Wales, 
this district mnst have been undergoing denudation almost un- 
interruptedly from the close of the Carboniferous period up to 
the present day—a length of time so vast that, bearing in mind 
what has been effected on the far later Miocene rocks, it would 
seem as though the Welsh mountains, hard as they are, must 
have been levelled with the sea. That no trace of any of the 
Jurassic or Cretaceous rocks now occurs in North Wales would 
be readily explained by their comparative softness, and the 
readiness with which their d4ris would be swept away in such a 
general shifting of surface accumulations as took place in the 
Glacial Period. 
During a portion of the Glacial Period the work of denudation 
was partially interrupted, and the valleys which had already 
assumed their present size and shape were more or less choked 
with very irregular deposits of boulder clay and gravel. From 
that time to this the work of the streams has been to clear out 
these obstructions from the old rock-valleys—a work that is not 
yet nearly accomplished. A remarkable instance of post-glacial 
denudation exists in the Vale of Gresford. The River Alyn, 
leaving the hilly region of Hope, winds across an undulating 
plateau of sand and gravel, which extends as far as the low- 
lying boulder clay plain of Rossett. In this plateau the Alyn 
has excavated a narrow but deep valley, reaching and cutting 
into the rock beneath; the material has been spread out by the 
river over many acres of the nearly level ground about Rossett, 
where the current lost its transporting power. And in many 
other cases the rivers flow in channels contained almost wholly 
in the glacial drift, which thus forms a protective covering to 
the rock beneath. When this has been removed, and the 
streams have once more regained their pre-glacial channels in 
the solid rock, the deepening of the valleys, so long interrupted, 
will be continued, until the next great terrestrial movement 
places North Wales once more beneath the level of the sea. 
