Miss Eyton — Drift-beds of Llandrillo Bay. 351 



present, both as to depth and area ; — covering the summits of hills 

 some 400 or 500 feet in height, and extending, probably, far out into 

 what is now the Irish Channel. Then came the period of denuda- 

 tion, when the land sunk beneath the waters of the Glacial ocean, 

 which, rising over these summits, and others much higher,^ washed 

 away the lighter material, leaving the heavy boulders standing — as 

 they are so often found — in their original position. 



It is by considering the miniature operations of the present sea, 

 that the conclusion is arrived at, that the denudation of the Boulder- 

 clay was mainly effected by an advancing, not a retiring, ocean. 

 The latter, having expended most of its force, appears to have little 

 influence in altering the surface of the land, though it leaves behind 

 it much of the material which it had previously accumulated and held 

 in suspension, but which it is unable to carry away. Its tendency is 

 rather to produce smooth slopes and rounded outlines, than sharply 

 defined cliffs, or any of the forms which are the result of under- 

 mining action. Therefore, while extensive denuding operations, 

 including the formation of caves and terraces, are marks of the " 

 sinking of the land, I conceive that the deposition of marine drift 

 may be considered a token of its rising, and of the consequent sub- 

 sidence of the water. 



2nd. Glacio-marine Drift. On the western side of Penmaen Ehos 

 the Boulder-clay thins out, and we find reposing upon it, against the 

 hill side, a differently constituted Drift ; while in the lower part of the 

 glen, near the village of Colwyn, the Boulder-formation has been 

 almost entirely removed and the same Drift substituted. Whether 

 this removal was effected entirely by marine agency ; or whether, 

 the land having risen, its surface was again ploughed by glaciers and 

 again submerged before the deposition of this second drift, there is 

 here no evidence to point out. The Drift is composed of small 

 pebbles, about the size of a pea, rounded and flattened, and closely 

 packed, mixed with sand, the whole being cemented together by 

 clay. It attains the greatest thickness towards the centre of the 

 glen, where it is cut into by a small brook, on the banks of which 

 many good sections may be found. In some places it is slightly 

 bedded. I never succeeded in finding any perfect organic remains 

 in this bed, but have frequently detected minute fragments of shells 

 among the sand. This is accounted for, if we suppose it to have been 

 subjected to Glacial as well as marine action. I found the same 

 Drift near the caves at Tanr'ogo. Further observation convinces me 

 that the interior of these caves, as well as of those of Cefn Ogo, near 

 St. Asaph, and the smaller ones on the beach at the foot of Penmaen 

 Ehos, are of far older date than the associated Drift, being simply 

 openings in the Glacial fissures into which the rising sea has 

 obtained an entrance, and has hollowed them into caverns with 

 arched entrances.^ Could the necessary means and permission be 



1 Sir Charles Lyell speaks of drift-beds containing marine shells on Moel Tryfaen, 

 1360 feet above the sea level (Elements of Geology, p. 158). 



2 Query. May not some of the bones found in these and similar caves have been 

 washed in through the fissures with the Boulder-clay, although the caves themselves 



