High-Level Marine Drift at Colwyn Bay

Abstract

abstract The drift-deposits of Colwyn Bay were described by the Author in 1885, but at that time he did not find any marine drift above the 200-feet contour. In the winter of 1897–98 he was fortunate enough to discover a mound of marine drift, consisting of sand capped with Boulder Clay, at a level of 560 feet above Ordnance datum. It is situated at a distance of about 1 mile south-by-west of Colwyn Bay railway-station, the longer axis running in a north-easterly direction, and measuring about 90 yards, the shorter axis being about 50 yards in length. It stands upon the watershed separating the valleys of Nant-y-glyn and Colwyn Bay. It is evidently an outlier of the marine drift at a lower level, consisting as it does of the same well-rounded grains of quartzose sand, and containing far-travelled erratics, among which are Eskdale and South of Scotland granites mixed with Welsh rocks, many of which, according to Mr. T. Ruddy, to whom they were submitted, came from the head of the Conway Valley. The interest of this find lies in the apparent isolation of the mound and its situation on the watershed at a high level. The intervening ground between the mound and the marine drift skirting the coast is occupied with blue Till, the product of land-ice, in which only Welsh rocks are found. High-level marine sands are found flanking the Vale of Clwyd on the Flintshire side, but the Author knows of no record of high-level marine drift in the neighbourhood of Colwyn Bay. There is no doubt that the formation of the mound is geologically subsequent to that of the blue Till. The Author further describes stratified marine sands containing shell-fragments, resting upon brown Boulder Clay, exposed in three sandpits near the Old Colwyn Road at Groes, on the east side of Nant-y-glyn. At Old Colwyn, on the east side of the valley, there is a great development of a similar sand lying between the 100- and 200-feet contours, also containing shell-fragments. A depth of 30 feet of this sand was exposed unbottomed, and it is probably as much as 60 feet deep. On the west side of the valley, below the 100-feet contour, an excavation for a house showed angular and subangular gravels of Welsh rocks containing a lenticular seam of sand with rounded grains, and this appeared to be a passage between the purely land-ice or fluvial drift and the geologically overlying marine sands. It is a notable fact that the marine sands of Groes, of Old Colwyn, and of the Vale of Clwyd lie on the east side of their respective valleys, while the marine Boulder Clays lie to the greater extent on the west side; and that the marine sands have mostly accumulated as bars near the mouths of the valleys.