﻿184 MR. W. G. FEARNSIDES ON THE [May 1910^ 



is possibly of late Glacial date, and may be due to floods coursing 

 along the edges of the waning valley-glaciers at a stage when they 

 were becoming rather short of, or perhaps free from, lateral moraine. 

 It is a phenomenon which is common to the southward-facing 

 slopes of the lower courses of many of the valleys of Carnarvonshire 

 and Merionethshire. 



Of post-Glacial accumulations Ynyscynhaiarn has its full share. 

 A raised beach I have not certainly identified, but the promontory 

 of Craig-ddu seems to show a wave-cut platform about 10 feet 

 above most tides, and at Ynys Gyngar, Careg-cnwc, Gareg-goch,. 

 Trwyn-y-borth, and Trwyn-cae-iago (Craig-y-don) about Borth, I 

 think that I can recognize a similar but less deeply cut wave-notch. 

 The various Ynys about Tremadoc also show wide rock-platforms a 

 little above the level of the morfa ; but it would require careful 

 measurement to determine their relationship to the tide-level outside 

 the embankments. 



The alluvial deposits of the Afon Glaslyn and the manner in 

 which tidal silt is building up the Traeth Mawr into land is 

 well known. The cliffs beyond Tan-y-graig below Craig Pant-ifan 

 have no scree at their base, and are pierced by caves as fresh now 

 as in the days when they were washed by the tide. The Roman 

 inhabitant of Hen Fynwent was a fisherman, and the find of kitchen- 

 middens of cockle-shells (Cardium edule) at Wern shows that the 

 siltirjgup of the whole morfa of Penmorfa is not much more ancient. 

 There was a sea-bathing place at Tremadoc at the beginning of last 

 century. 



The morfa of Morfa-bychan is of similar date, but owes its 

 formation to the sand blown in from the tidal sands and to the 

 growth of salt-marsh plants in the bogs behind the sand-hills. 



The pebble-bank between Criccieth and Craig-ddu is also worthy 

 of notice. It is formed by tidal drift from the west, and consists 

 of well-sorted rounded pebbles from the Western Drift-mass of 

 Criccieth ; and, as in their eastward migration the pebbles are 

 unable to pass Craig-ddu, it is still growing. Pushed out as a 

 spit from the cliff of Ogof-ddu, the beach has barred the outflow of 

 the waters which come from the hills into Llyn Ystumllyn, and 

 holding up the lake has compelled its waters to find exit between 

 the pebbles or through the well-jointed rocks of Craig-ddu, which 

 they do most effectually, appearing at low tide as springs on the 

 foreshore. 



XVII. General Summary and Conclusions. 



The district of Ynyscynhaiarn consists of Cambrian and Ordo- 

 vician rocks arranged about a northward-pitching north- and-south 

 anticline, which has its axis through Llyn Ystumllyn close by 

 St. Cynhaiarn's Church. The rocks have been cleaved along lines 

 which range a little east of north and south, dip eastwards, and 

 are broken by a series of thrust-planes, which are inclined to the 

 north-east. Into the sedimentary rocks, possibly along the lines of 



