884 REPORT — 1898. 



cutting out and dispersing the material, the banks being restored by tidal action 

 in calm weather and during oflF-shore winds. The action of waves due to wind is 

 intermittent, Tariable in direction, and irregular. The travel of shingle, except 

 when acted on by gales, is continuous, regular, and constant in direction. It is 

 shown by a number of examples that the travel of shingle is not coincident either 

 with the prevailing or predominant winds, but on a tidal coast the predomi- 

 nant drift is invariably in the same direction as that of the flood tide. The action of 

 the tides in heaping up and drifting material is due to wave action. The rise and 

 fall of the tide on the coast does not consist of a mere vertical rise and fall of the 

 water, but of a continual oscillation. The crest of the tidal wave in the open sea, 

 being in advance of that near the shore, results in an oblique lateral movement 

 along the beach, and the advance of the water being checked by the shallow bed 

 with which it comes in contact is reflected back, resulting in a series of small 

 oscillations or waves which break when they reach the low-water line. These 

 oscillations are ever present on the margin of the shore, even when the sea is 

 calmest, and are never absent except when absorbed by larger waves due to gales. 

 These tidal wavelets vary in height from 6 inches to 2 feet, and break on the shore 

 at the rate of from ten to twenty a minute according to the rise of the tide and 

 the slope of the beach. These wavelets, aided by the flood current, lift up and 

 carry forward any coarse sand, loose stones, or other material with which thev 

 come in contact, and leave some portion of it stranded at the highest point to 

 which the tide of the day reaches. The wavelets, besides lifting and transporting 

 the shingle, brush upward the whole of the face of the bank, and gradually raise 

 it above the line of high water. It is shown that, though these waves are small, 

 they by their weight and velocity develop sufficient force to move a large quantity 

 of pebbles. A wave having a height of only a foot from trough to crest, giving a 

 head of 6 inches, and containing a volume of water equal to a weight of -142 ton, 

 has sufficient kinetic energy to raise 165 lbs. of pebbles a foot high. Allowing the 

 weight of pebbles in water to be 100 lbs. to the cubic foot, each wave, if the whole 

 of its energy be applied to the movement of the material, is capable of raising 

 660 pebbles 2 inches in diameter a foot ; or, with fifteen waves to the minute, 

 9,900 pebbles a minute and 2,376,000 in a single tide, or a total weight of stone 

 of 266-4 tons a foot high. This, however, is beyond the work actually done, as a 

 portion of the energy of the wave is absorbed in friction. The above rough 

 approximation of the power of the wavelets is sufficient to show the enormous 

 power that is developed by tidal action day by day on the coast, and the capability 

 of the wavelets due to the tides for building up shingle banks and drifting the 

 pebbles along the beach. 



4. Further Exploration of the Ty Newydd Cave, Tremeirchion, North 

 Wales. By Rev. G. C. H. Pollen, S.J., F.G.S. 



In a paper read before the Geological Society on December 15, 1897,^ the 

 author gave the results of the exploration of 60 feet from the old quarry. The 

 work has now been extended to 150 feet in this direction. In the upper portion 

 a stalagmite floor has been found in situ, completely sealing up the local gravels. 

 Over this were found 5 feet of clay with broken limestone, which is all that is left 

 to represent the strata which previously formed the roof of the cave. The 

 whole is now overlaid with boulder clay, containing many specimens of northern 

 and western drift, with striated stones of more local origin. No trace of erratics 

 or of glaciated stones have been found in the lower cave materials. 



The cave has also been traced for 55 feet across the floor of the quarry where 

 it re-enters the rock, running in the direction of the gully which separates it from 

 the Ffynnon Beuno and Cae Gwyn Oaves. In the lowest gravel of this part a 

 water-worn fragment of the molar of Equus was found. The following succession 

 seems to be established for the contents of the cave : — 



' Q.J.6.S., February, 1898, pp. 119-134, pi. viii. 



