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CAVES AND CAVE DEPOSITS. 173 
Laminated clays are not evidence of glacial action, but only 
of alternations of muddy and clear water, such as follow rainy 
and fair weather. 
Some of the most interesting caves, in respect of their con- 
tents and the light they throw on the history of primeval 
man, are only rock-shelters—abris—such as are seen in the 
Dordogne district, and are common in our district along the 
valley of the Elwy. They are sometimes longitudinal sections 
of parts of subterranean watercourses, but are more commonly 
due to the weathering away of soft rock between two harder 
beds. It does not always require a stream or direct rainfall 
to wet the surface of a rock. The travelling moisture of 
the air, condensed in and on the cold rock, is enough, and is 
probably the chief agent in case of a rock undercut so far that 
the rain cannot touch it, just as Rendu explains the film of ice 
upon the snow at high elevations, not by the melting and re- 
freezing of the snow, but by the condensation of the little 
moisture left in the air which comes in contact with the snow in 
those high regions. 
Turning now to the Vale of Clwyd, we find there many caves 
representing subterranean channels, through which the water 
running off the Silurian hills around dropped into fissures on 
reaching the mountain limestone, and following now the faults, 
now the joints, and now the bedding, found its way down to 
the lower ground. This was, of course, a process which extended 
over a long time. While the main valley was being scooped 
out the tributary gorges were being cut back, and so the under- 
ground waterconrses kept finding lower outfalls, leaving their 
old channels as dry caves, It is clear, therefore, that the caves 
are of different ages, and the evidence of their antiquity must 
be sought in the physical geography of the district. The age of 
their contents is a separate question. The Plas Heaton cave 
seems to belong to a very remote period, and the stream that 
ran through it had been cut off by later denudation either in the 
gorge of the Elwy, or in the valley that runs into the Elwy below 
Llysmeirchion. Yet the deposits of Plas Heaton cave have not 
yielded evidence of very great antiquity. Some of them indeed 
must have been washed in in quite recent times, for I found 
pieces of old magnums in the cave earth beyond where man 
could creep when first I knew the cave. The other end of this 
cave was blocked with clay full of boulders—a mass indis- 
tinguishable from the clayey Clwydian drift. The Pontnewydd 
Cave is one of the numerous old watercourses which represent 
the ancient subterraneous drainage of the Elwy valley. It 
Occurs some way down the precipitous slopes of the Elwy 
behind Cefn. The other end of that cave has not yet been 
found. In it were felstone implements and flint flakes, and a 
mass of stuff washed in from either entrance, and along all 
fissures which communicated with the once drift-covered surface 
above. 
D2 
