Summary

Daniel Mackintosh Quarterly Journal of the Geological Society of London (1876)

The angular limestone fragments may have fallen from the roof or sides of the caves during the period of accumulation; or previously fallen fragments, along with the bones of animals, may have been washed up into the clay by the waves of the Upper-Boulder-clay sea. It ought not to be forgotten that in caves sea-waves are often possessed of very great force, and that they are capable of insinuating themselves into the remotest recesses.
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Daniel Mackintosh Quarterly Journal of the Geological Society of London (1876)

However it may have originated, I cannot believe with Professor Hughes that, in the Pont-newydd cave, it was washed in through a swallow-hole, from the Boulder- clay of the neighbourhood, by a freshwater stream. The relative proportion of stones of different kinds is not nearly the same in the pebble-bed and in the Boulder-clay covering the surface of the ground at a higher level. In the pebble-bed nearly all the stones, so far as I could see, are Denbighshire sandstone or grit. In the Boulder-clay a large proportion of the pebbles are limestone.
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Daniel Mackintosh Quarterly Journal of the Geological Society of London (1876)

The bed under the stalagmite in the Cefn cave, with its representative in the Pont-newydd cave, is the most difficult to explain. I am unable to say whether it most resembles an accumulation of fine river-shingle or a raised beach. In the caves it does not occur at the height above the present sea-level of the more typical raised beaches.
Source: Wikisource

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