XC PEOCEEDIly^GS OF THE GEOLOGICAL SOCIETY. [May 1 898, 



or that remains of extinct mammalia were washed into them before 

 any of the Drift from the western and northern areas, now found 

 so abundantly spread over the ground above and in the neighbour- 

 hood of the caverns, could have reached this area. The caverns 

 themselves were entirely filled by local materials which must have 

 been deposited in them by flood-water at the very commencement of 

 the Glacial period. As these caverns are situated in the same ravine, 

 but on the opposite side to those of Ffynnon Beuno and Cae Gwyn, 

 I think it may be well to repeat the conclusions arrived at by 

 Mr. PoUen (Quart. Journ. Geol. Soc. Peb. 1898, p. 132) :— 



' 1. The material in the Ty ISTewydd Caves, and in the lower part 

 of Efynnon Beuno and Cae Gwyn, is of purely local origin. Of this 

 we can speak with confidence, as the question was before us from 

 the beginning ; we have, therefore, examined all the gravels with 

 minute care, and all stones of whose origin we did not feel certain 

 were forwarded to Dr. Hicks. 



' 2. This local deposit is of earlier date than the Boulder Clay 

 with western and northern erratics. This was sufficiently proved 

 by the occurrence of the granites and felsites on the hillside at a 

 much higher level than the caves. All doubt on the subject is, 

 however, now removed by our having found the two beds actually 

 superimposed in the second vertical shaft. 



' 3. The occurrence of the rhinoceros-tooth shows that there was 

 a land-surface, and a climate capable of supporting such large 

 mammalia, either before or during the period when the cave was 

 being filled.' 



The height of the caverns above sea-level is 422 feet, or about 

 20 feet above the floor of the Cae Gwyn Cave. 



The Caves on the West Side of the Yale of Clwyd. 



"^'hen referring to these caves, Prof. Hughes, in his paper,^ says 

 that the so-called Cefn Caves in the Elwy Yalley ^ are obviously 

 due to the decomposition of the limestone along the weaker lines in 

 the general drainage-system of that valley ; but what the particular 

 local conditions were that caused the subterranean channel to 

 plunge down suddenly to an outlet far below, is not so clear.' Of the 

 Plas Heaton Cave, he says that it ' must have been formed under 

 quite different conditions. It does not lie in the line of any existing 

 drainage-system ; it must be a very ancient cave ; perhaps it was 



1 Quart. Journ. Geol. Soc. vol. xhii (1887) p. 103. 



