Quarterly Journal of the Geological Society of London/Volume 32/On the Correlation of the Deposits in Cefn and Pont-newydd Caves with the Drifts of the North-west of England and Wales

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11. On the Correlation of the Deposits in Cefn and Pont-newydd Caves with the Drifts of the North-west of England and Wales. By D. Mackintosh, Esq., F.G.S. (Read June 23, 1875.)

In the present state of Posttertiary geology it is of very great importance (as may be inferred from the Presidential address just published, May 1875) that some one should attempt to correlate the deposits in caves with the glacial drifts of the neighbourhood. I therefore venture to bring before the Society a brief statement of the results of observations lately made in and around the Cefn and Pont-newydd Caves, Denbighshire. These caves are situated near to each other in the face of a limestone escarpment, on the north bank of the river Elwy. The Pont-newydd cave has been described by Professor M'Kenny Hughes and the Rev. D. R. Thomas, in the Journal of the Anthropological Institute (vol. iii. p. 387), and by Mr. W. Boyd Dawkins, F.R.S., in his work on Cave Hunting. By these writers the cave-deposits are regarded as Postglacial. The best account of Cefn Cave, as it existed before the deposits were nearly all cleared out, is perhaps to be found in Mr. Joshua Trimmer's 'Practical Geology,' published in 1841[1], the following being the order of succession therein stated or implied:—

1. Sand, silt, and marl, with sea-shells in one or more places (uppermost).

2. Loam, with angular fragments of limestone and bones, filling the cavern nearly to the roof {diluvium of old authors).

3. Crust of stalagmite.

4. Loam, with smooth pebbles, bones, teeth, and fragments of wood (lowest).

Mr. Trimmer believed that the lowest of these deposits was introduced, before the glacial submergence, by the adjacent river while flowing at a considerably higher level than now. But that the river-channel must then have been excavated to a level as low, if not lower, than at present, is evident from the fact that the Upper Boulder-clay extends down to, and in some places runs under, the river-bed, in a manner showing that here (as elsewhere in the north-west of England and Wales) the river, since the glacial submergence, has been principally occupied in reexcavating its choked-up channel. Trimmer likewise believed that the deposit with sea-shells was introduced by the sea through a fissure in the roof of the cave.

From the nature and sequence of the deposits in Pont-newydd cave (a considerable portion of which has not yet been cleared out), compared with what I have seen of the remnants still visible in the Cefn cave, and from a further comparison of the facts thus obtained with accounts given by T Mr. Trimmer, Mr. J. Price, M.A. (of Chester), Professor Hughes, Mr. Boyd Dawkins, &c., I have been led to regard the following as the sequence of the beds (order descending):—

1. Coarse sand charged with minute fragments of sea-shells, still found adhering to one side of a rising branch (ascended by steps) of the Cefn cave[2]. In the Pont-newydd cave a bed of very fine stoneless clay.

2. Clay, with angular and subangular fragments of limestone, a few polished fragments and pebbles of limestone, likewise a few pebbles of Denbighshire sandstone and grit, felstone, &c. This deposit (which contains bones of a number of the usual cave Mammalia) is horizontally continuous with the Upper Boulder-clay of the district (see sequel).

3. Stalagmitic crust, from less than an inch to 2 feet in thickness. Very little now left in the Cefn cave; apparently absent in the Pont-newydd cave.

4. Loam, with rounded and smoothed pebbles, bones, teeth, and fragments of bone and wood, in the Cefn cave. In the Pont-newydd cave a bed of extra-rounded pebbles, more or less cemented by stalagmitic matter.

The coarse sand in the uppermost bed (1) in no respect resembles the Upper Boulder-clay on the summit of the plateau above the cave; it is not what would result from a subaerial rearrangement of the clay; and the proportional number of fragments of shells is very much greater than that found in the clay[3]. It is therefore probable that the coarse sand was introduced by the sea through a fissure or fissures in the roof, as Trimmer supposed.

The clay (2) can be traced along the plain of Lancashire and Cheshire, the coast of Flintshire, and up the vale of Clwyd. It spreads over the gently rising ground between St. Asaph and the Cefn and Pont-newyd caves; and it may be seen all around the caves, in some places filling up hollows, in others covering plateaux, and in not a few instances clinging to the face of steep slopes, or even adhering to narrow rocky terraces or ledges. I have been familiar with this clay in Cheshire and Flintshire for four years, and have therefore little hesitation in asserting that traces of it, in an unmodified state, may be found at the entrances of both the Pont-newydd and Cefn caves—that in the interior of the Cefn cave, for a considerable distance from the entrance, there are indications of this clay having once filled the cave nearly, if not quite, to the roof—that in the interior of the Pont-newydd cave it maintains its unmodified character for a considerable distance from the entrance—and that in no part of these two caves has this clay been modified further than what may have resulted from the dropping of calcareous matter, from the temporary ponding back of water in the recesses or hollows, or from accumulation within the caves under conditions which may have differed from those without. 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.

The stalagmitic crust (3) must have been accumulated during a period when conditions were favourable; and these conditions must have varied in different parts of the Cefn cave.

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. It is not confined to the caves, but may be seen at intervals along the sloping north bank of the Elwy. It may likewise be found in patches along the Elwy valley as far as the Vale of Clwyd, where it would appear to graduato into the middle gravel and sand which underlies the upper clay. 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. I could only see one felstone specimen in the pebble-bed (though others probably might be found). In the Boulder-clay felstone pebbles and boulders are far from being rare[4]. The pebble-bed is not confined to the cavern or the ground straight in front of it, as it would have been if deposited by a stream flowing out of the cavern; but it may be seen in a recess a short distance east of the mouth of the cavern, and, as I have already remarked, at intervals further east. Several examples of it may be found near the Cefn cave, clinging to the rocky slope. This bed could not have been accumulated before the glacial submergence, as it contains a few erratic pebbles[5], which must have been transported by ice from regions far beyond the basin of the river Elwy.

Before endeavouring to offer a general explanation of the mode in which the various deposits were introduced into the Cefn and Pont-newydd caves, it may be necessary to state that the drift- deposits of the north-west of England and the borders of North Wales are separable into:—(1) a lower stony Boulder-clay (with glaciated stones), which to the south of the Mersey and in the eastern part of Wales can only be detected at intervals; (2) a middle sand and gravel (without any large boulders or glaciated stones, excepting a few among or near to the mountains), which extends almost continuously over very wide areas, and often attains a thickness of nearly 200 feet; (3) an upper Boulder- or brick-clay (with glaciated stones and a few boulders), which is quite as persistent as the sand and gravel, but which has not yet been clearly traced to a height of more than a few hundred feet above the present sea-level. The first indicates a cold period, the second a comparatively mild period, and the third, I believe, a second cold period, though this has been disputed by Professor Hughes and Mr. Kinahan. Believing with Professor Ramsay that the great glacial submergence commenced before the ice-sheet or ice-sheets disappeared from the country, that the lower Boulder-clay (so far as it is of marine origin) was accumulated while the land was sinking, and that the sand and gravel formation was deposited while the land was rising—and likewise believing in an interglacial period, during the first part of which the land was still submerged, dry land prevailing during the second part, it follows that there must have been a second submergence, during which the upper Boulder-clay was deposited. It is possible that the pebble-bed in the Pont-newydd cave may have been deposited as the land was rising out of the interglacial or Middle-Sand-and-gravel sea, and that pebbles and loam may then have been introduced into the Cefn cave; but as the neighbourhood could not have been inhabited by land animals until after the emergence of the land, the bones, teeth, and fragments of wood which were found associated with the lowest deposit in the Cefn cave may have been washed in by rain through fissures in the roof. After the accumulation of the stalagmite, more bones must have been introduced, and the cave may have been temporarily inhabited by the hyæna. This state of things may have been brought to a close by the submergence of the cave beneath the waters of the Upper-Boulder-clay sea, which filled the cave nearly (if not, in many places, quite) to the roof. The sand with sea-shells may have been introduced through fissures in the roof while the plateau above the cave was gradually rising above the sea-level.

The above imperfect explanation is the best I can offer concerning the mode in which the different and very dissimilar deposits in the Cefn and Pont-newydd caves were accumulated.

P.S.—As may be learned from his paper read before the Anthropological Institute, Professor Hughes found Palæolithic flint implements and a human tooth, which he believed came from the bed I have called Upper Boulder-clay, in the Pont-newydd cave[6].

  1. See also paper by Rev. E. Stanley, Proc. Geol. Soc. vol. i. p. 402.
  2. I had no difficulty in raking off large quantities with a hammer. It would appear to be present in at least one other branch of the cave.
  3. I could see none at all in the clay of the immediate neighbourhood of the cave, though they probably might be found, as I believe Prof. Hughes has collected them from the same clay in the Vale of Clwyd.
  4. It likewise contains Eskdale granite.
  5. Professor Hughes mentions the occurrence of felstone in this bed in Pont-newydd cave. I saw a large boulder of very typical Arenig felstone, about halfway between Denbigh and Cefn; and I believe that a boulder which may be seen a short distance N.W. of the Cefn cave must have come from the same mountain.
  6. Since this paper was written, Professor Hughes has found a large fragment of felstone in the lowest bed of the Pont-newydd cave.