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Rhyl Church Guild v. Prestatyn.



GEOLOGY (continued).


GEOLOGY (continued). The ancient line:of cliff against which the deposits are banked, as described above, is breached at Dyserth by a steep-sided pre- glacial valley, up which there runs a tongue of boulder clay from the plain to the higher ground, and spreading out over the uplands. The type of boulder clay seen on the coast is found to give way inland to a local till of Silurian debris. The newest member of the drift is a tough, homogeneous, chocolate-coloured clay, with few boulders, consisting of grey granite, por- phyries, limestones, quartz and shell frag- ments. It is worked for bricks near Meliden. The borings and sections show that it is interstratified with eaud beds, though these do not often appear at the surface. At Pres- tatyn there is a considerable area of soft red sand, and from Dyserth Hall a similar sand with. interstratified beds of well rounded shingle extends through Bodrhyddan Park 0 southwards. The boulder clay is well seen in the valley at the foot of Dyserth Castle Rock, and a little to the north of the stream which runs into Dyserth Waterfall, a deep tongue runs up what appears to be the pre-giacial line of drainage, separating the roek-hills of Craig Heilyn and Dyserth Castle. At the top of the limestone plateau this boulder clay spreads out from one to two miles in breadth, and runs for some miles to the south. A. good section has been opened near the terminus of the Prestatyn and Dyserth branch railway, where the clay has been cut into for •; depth of 18 feet without being penetrated. i It ttt of a deep red colour, similar to the drift bf tire plain. At Gwaenysgor, Gop hill, and Axton, the red tint predominates in the tongues of boulder clay, due to the clay being composed of the insoluble residue of limestone. A red tint often prevails in the soil of the rocks. Sand and gravel appears at intervals from beneath the clay at these high levels, as in the plain. At Henfryn the Newmarket brook has cut through red boulder clay, and exposed sand with shell fragments, and at Hendre-mynach there is 25 feet of sand under a yellow and more gravelly clay. In most cases the sand and gravel pushes up through tho Boulder Clay in the form of steep mounds or ridgos. A prominent example occurs near the Prestatyn nnd Dyserth railway terminus close to the clay before alluded to. A steep ridge of gravel l'ises boldly up and for about 800 yards in a ULE. direction along the centre of the valley. The bedding of the gravel is arch-shaped, so as to bo roughly parallel to the shape of the ground. The boulders consist of 60 per cent of limestone, the larger ones being about six inches long, the remainder being chiefly Silurian. The coarse gravel is interstratified with bands of grit and sand with shell frag- ments, and the whole is cemented into irregu- lar blocks by carbonate of lime. Similar gr&vpl-banks occupy the valley again a short distance to the north-east, but with an increase in the proportion of lime- stone boulders to 90 per cent. A sand-pit 500 yards S.S.W. of the Gop Farm, show the superposition of the red boulder clay on the sand. The clay contains many scratched blocks of limestones about one foot long, a large boulder of voleanic ash from the Aronig mountains, and a few-shell-fragments. The sand is fine. with gritty patches and veins of loam or clay, and also contains shell-frag- ments. The bedding of the sand is disturbed 'immediately under the clay. An esker-lik,e development of sand and gravel appears again at the margin of the boulder clay on the east side of Moel Hira- ddu-g. The ridges consist of stratified sand and gravel with shell fragments, but after running for 400 or 500 yards die out, leaving the clay in contact with the rock. North of Gwaenysgor a long strip of sand and gravel is shown running over the black limestone, and Chert outcrops towards Kelston. The line of separation between this deposit and the boulder clay is very indefinite. A gravelly character invades the clay, and chert-fragments increase in abundance north- wards. Finally the gravelly clay passes into a sand and gravel deposit, which forms characteristic banks near Gulgrave Hall. The same difficulty is met with in drawing a boundary between the sand and gravel and boulder clay about Axton, though the former preserves in the main its habit of rising into banks and hillocks. One of these occurs near Pant-Glas, with an axis running about E. IQo S., and nearly parallel to the valley. Scattered over the eountry, over the drift- covered areas and bare hill-tops alike, there occur large erratic blocks. Though scarce in the area now specially under consideration, they become exceedingly abundant further south, where they are found up to an altitude of more than 1,000 feet. A very large proportion of these have been derived from the felspathic volcanic series in the Bala beds, of the Snowdon and Arenig groups of mountains, and have therefore travelled in a N,E. direction. There is one measuring 5x4x8 feet on the Cwm mountain, 880 feet above Ordnance Datum. Another measuring 9x6x5 feet, lies in the bed of the stream near Hendre-bach (500 feet). Another measuring 3 X It X 1 ft. lies on the bank of the stream above Hendre- fawr (600 feet), and is possibly from the Bala beds. Another is embedded in the side of the road at Pentre, near Newmarket, but is smaller than the others. These felspathic boulders are confined generally to the areas in which drifts of western derivation occur.



The League.