6 
NOTES ON THE GEOLOGY OF 
similar conditions to what we had at the commencement of the 
Carboniferous Period. 
There is not much written on the Geology of the Vale of 
Clwyd. Ramsay Mem. Geological Survey, Vol. iii., devotes 7 
pages to it (222-8); a few other Papers on special points will 
be noticed in their proper place. 
There are five plains distinguishable from the hills that 
bound the Vale of Clwyd— 
No. 
No. 
1.—The plain of the Carnarvonshire mountains, having 
an average height of about 3,000 feet, and probably part 
of the same plain as that of the Lake District mountains - 
which attain about the same average elevation. 
2.—The plain of the Denbigh Grits and Flags which 
rises to the S. and W., and may have been continuous 
with the 2,000 foot plain of South and Central Wales. 
This is the plain that you see stretching from the 
Black Mountains in Carmarthenshire right up to the 
great North Wales group, mentioned above, No. 1; 
interrupted by Plinlimmon only, which mountain must 
have formed an island when the sea was planing the 
old land down from the 3,000 to the 2,000 foot level. 
This is probably part of the second plain of the Lake 
District, namely, the plain of the Carboniferous moor- 
lands of Western Yorkshire and Northern Lancashire, 
which cuts across the Silurian of the Howgill Fells, and 
at an average level of something under 2,000 feet, carries 
the eye along to the ancient shore cliffs of the higher 
Lake Mountains. 
There is a suspicion of another level, at about 800 feet, 
both in the Lake District and in North Wales, but 
I have not been able to make it out satisfactorily. 
. 3-—Within the Vale of Clwyd (see fig. 1) three other 
plains can be made out. They might be called terraces, 
but two of them are probably marine, and traceable into 
other and larger areas where they may fairly claim the 
title of plain. The highest is the plateau on which Plas 
Heaton stands. It is generally covered by more or less 
continuous patches of the Clwydian marine drift. It was 
the bottom of the valley before the gorge of the Elwy 
was excavated. 
No. 4.—There is below this, at a level of about 80 feet above 
the sea, the level surface of the old estuary of the Clwyd, 
on which the Cathedral of St. Asaph stands. This seems 
a plain of deposition, and the low flat hills and spurs 
which have been left between and on either side of the 
modern streams of the Clwyd and Elwy are composed of 
the estuarine drift described hereafter. 
