DRIFTS OP THE YALE OF CLWYD. 



103 



There are many kinds of caves, and there are many different ways 

 in which caves are formed, and it is impossible to investigate their 

 age without inquiring somewhat into the geology and physiography 

 of the district ; but I shall dismiss this part of the subject very 

 shortly with a reference or two. 



First, we are not dealing with artificial caves in any case, nor 

 with caves formed by the sea. The cliffs in which they occur are 

 not sea-cliffs *. We have to do with natural caves, such as rock- 

 shelters due to the subaerial wasting away of parts of the rock 

 which were readily shelled off under the influence of variations of 

 temperature and of the moisture which was condensed upon it. 



The most favourable condition for the formation of caves is that 

 the limestone should have been exposed to the action of the weather 

 and its joints opened out, and that then it should be partially covered 

 up by boulder-clay, which would collect all the water into runlets, 

 and so concentrate it upon certain lines of weakness and form 

 underground watercourses f . 



Still more common than the rock-shelters, and more important 

 for our present inquiry, are the caves which represent ancient sub- 

 terranean watercourses in the limestone rocks which flank the vale 

 on either side. How such caves were formed is well known ; but 

 when they were formed, and to what drainage-system they belong, is 

 not always clear. 



For instance, in the Elwy valley the Cefn caves 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. 



The adjoining Pontnewydd cave, so far as yet explored, runs 

 approximately on one level. 



Near the top of the opposite hill the Plas Heaton cave 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 formed when the streams that flow 

 down near Llysmeirchion ran at a greatly higher level, the inter- 

 mediate ground being all filled up with drift, or perhaps when the 

 drift choked up the Elwy valley, as we have seen above was once 

 the case, some of its waters may have found their way into the lime- 

 stone rocks above Plas Heaton. 



If the Cefn caves were formed by water collected on the imper- 

 vious drift, then we must refer them to a later date than the Plas 

 Heaton cave ; but this is not quite certain, as they may have been 

 partly formed when the gorge was first being cut back, and the 

 stream ran partly underground, as now in many limestone valleys, 

 and only in flood ran along the channel of the apparent stream- 

 course. More probably some, if not all, of the Cefn caves were formed 

 much later, when the gorge was filled with boulder-clay, and the 

 water ran into swallow-holes along the margin of the drift and rock, 



* See Whitaker, Geol. Mag. vol. iv. 1867, pp. 447, 483. 



t See Prestwich, Quart. Journ. Geol. Soc. vol. x. (1854), p. 222. 



