104 



PROP. T. M'KENNT HUGHES ON THE 



and part of such a one became a sloping cave. In the submer- 

 gence this cave cannot have been formed, as nothing would make 

 the current fall to open out vertical passages at the bottom of the 

 sea. 



Caves of various age occur along the limestone hills that bound 

 the western side of the vale. Some are old fissures enlarged and 

 filled with various minerals, and being now opened out again, as 

 they are brought within the reach of surface-denudation. Such are 

 the water-caves and lodes exposed by mining along the northern 

 slopes of Cefn Meiriadog. 



Further north, upon the coast at Llandulas, caves, now left high 

 and dry in the cliffs above the sea, point to different geographical 

 conditions there ; while a little inland the travertine below Cefn- 

 yr-Ogof tells of the same process carried on to later times along the 

 margin of the drift-covered hills. 



Crossing to the eastern side of the vale, the fissured cavernous 

 limestone of Gwaenysgor and Dyserth has only less conspicuous 

 caves, because there are smaller areas of impervious beds above on 

 which the water could collect in streams. 



But near Tremeirchion the conditions are more favourable. About 

 half a square mile of limestone rising abruptly from the valley is 

 surrounded on the east and north and south by the Silurian hills, 

 and the water in every little ravine where it touches the limestone 

 tends to form swallow-holes and caves. The drift, too, overlaps the 

 limestone and carries the water further on to it in places. When 

 the gorge was deepened, the old caves were deserted, and the 

 streams burst out at lower levels. Such caves are quickly choked ; 

 for the hills are steep, and there is much drift and loose super- 

 ficial debris being washed down. 



One such ravine which runs down from Y Graig by Pfynnon Beuno 

 is full of caves on either side. Some probably were formed when 

 first the water touched the jointed rock in the bed of the little 

 stream. Some in much later times were fed by swallow-holes along 

 the margin of the drift, which was and is still being eaten back by 

 surface-denudation. JSfone of these are necessarily very ancient 

 caves, as we inferred the Plas Heaton and Cefn caves must be. The 

 upper cave, known as the Cae Gwyn cave, was probably fed by a 

 swallow-hole on the margin of the drift above the upper entrance ; 

 and from the lower cave you can now look out at the sky above 

 through an ancient swallow-hole which caught the water off the 

 drift-covered slope. 



In the valley of the Chwiier, a little further south, we have an 

 interesting proof that the waste of the limestone rocks and the 

 formation of subterranean watercourses was still going on long after 

 the sandy drift was formed. 



Near Caerwys Station, behind the inn which takes its name from 

 the white water of the pool, Pwllgwyn, there is a section through a 

 great mass of travertine which has been deposited against a mound 

 of drift. The travertine is shown to be newer than the red sand 

 and gravel, and to have been deposited against a steep slope of it, 



