THE PLKISTOCKNK SKIMKs 631 



Cheshire plain into Shropshire and over the lower parts of North 

 W;iles up to heights of 400 and 500 feet above the sea ; while 

 accumulations of sand and gravel with marine shells occur at still 

 higher elevations, up to heights of more than 1 200 feet on both sides 

 f Walr>. Tim-, on the summit of Moel Tryfaen, a hill overlooking 

 Carnarvon Bay, with a height of 1350 feet, there is an extensive 

 deposit of stratified sand and gravel containing marine shells, more 

 than sixty species having been obtained. Among the most abund- 

 ant of these are Tellina balthica, Gardium edule, Astarte borealis, 

 Cyprina i*litn<Hcn, and Turritella communis. It is irregularly 

 overlain by boulder-clay, and both deposits contain erratics from 

 Cumberland and Scotland, as well as Chalk -flints which came 

 probably from Ireland. These flints, as Mr. Mackintosh observes, 

 have travelled to a higher level than their source, for the highest 

 parts of the Irish chalk are between 900 and 1000 feet. 11 



Similar shell-bearing gravels have been found by Mr. Mackintosh 

 at heights of 1000 to 1230 feet on the east side of the hills near 

 Minera in Denbighshire; here also Eskdale granite, Cumberland 

 felstones, and Chalk-flints are mixed with the debris of local rocks, 

 and all are well rolled and rounded as if they had formed part 

 of a sea beach. Another similar deposit of shelly sand and gravel 

 ocelli's at Gloppa, near Oswestry. Within this Welsh area, how- 

 ever, we find tracts and patches of another kind of drift, which 

 is clearly the product of Welsh ice, and wherever the two are in 

 juxtaposition the local drift always lies underneath the deposits 

 left by the northern ice. This is well shown in the cliffs of 

 Colwyn Bay, where the following succession is visible : 12 



|" 

 - 



4. Brown boulder-clay with broken shells and some 



Feet. 



Newer 



northern erratics ...... 8 to 12 



- 



1 3. Obliquely stratified sands resting on an eroded 



surface . . . . . . . . 6 to 12 



I" 2. Cream-coloured till in discontinuous patches . to 2 

 Older-! 1. Hard bluish-grey till packed full of stones derived from 



the Carboniferous and Silurian rocks of Wales seen for 6 



.M"-t of Central and Southern Wales was covered by another 

 ice-field, its centre of accumulation being the high ground which 

 extends from Plynliinnum southward and eastward through parts 

 of Cardigan, Brecknock, and Carmarthen, and rises in many 

 places to over 1800 feet. The thickness of the ice generated on 

 this area was such that the ice-flow was only partially turned 

 aside by the great mountain masses of the Brecknock Beacons 

 and the Black Mountains, and was actually able to override the 

 northern escarpment of the South Wales Coalfield. Proof has 



