156 Henry Dewey — -Land-forms in Caernarvonshire. 



line, leaving parts of a district immune from attack and reducing at 

 the same time adjacent areas of similar geological formation and 

 structure to a featureless plain. In the present instance I reject 

 the hypothesis of subaerial erosion. Marine erosion proves its 

 capability of levelling rocks of all degrees of hardness, as anyone 

 familiar with coasts bounding the Atlantic must acknowledge. The 

 Cornish coast is a convincing instance of the sea's power to produce 

 level tracts, and moreover lands of all degrees of hardness ultimately 

 yield and become reduced to the limit of erosion. Such a marine 

 plain is seen at low-water spring tides near Bude, where a quarter of 

 a mile of bevelled rocks are exposed. They consist of alternate beds 

 of hard sandstone and slate, but all have been planed down to 

 a uniform level, or rather a long gentle slope towards the deeper 

 waters. The tide in rising suddenly covers this plain, and is apt to 

 cut the unwary off from retreat. On this coast it is always well to 

 remember that — 



Far back through creeks and inlets making 

 Comes silent, flooding in the main. 

 There can therefore be no valid reason to offer why the sea did not 

 similarly operate on this plain in !N^orth Wales. 



It is more difficult to determine the period when this reduction 

 occurred; the fact that the feature terminates in both Cornwall and 

 in North Wales at precisely the same height above sea-level suggests 

 that the two plains are contemporary, whatever their geological age 

 may be. That in North Wales the plain is pre-Glacial is proved; in 

 Cornwall there are strong reasons for supposing it to be of Pliocene 

 age. We are then perhaps justified in accepting as of the same age 

 the North Welsh plain at this level. 



In both districts, however, there are wide tracts at lower levels,^ 

 notably that at 200-300 feet above Ordnance Datum, but in both 

 these do not occur above 430 feet, with the exception of those 

 already mentioned at 700, 1,000, and 1,250 feet respectively. The 

 lower plains may also represent other marine plains or peneplains, 

 but with these I am not concerned. The point to be emphasized is 

 the occurrence in both districts of a plain which does not rise higher 

 than 430 feet above sea-level. 



Before concluding I wish to express my thanks to Mr. Greenly for his 

 kindly advice and suggestions made during the writing of this paper. 



CoNCLtrSIONS. 



1. Glacial phenomena as expressed in land-forms have long been 

 known in North Wales. In the valley of the Ogwen the whole series 

 of land-forms characteristic of glacial topography are represented, 

 namely, the lakes, cwms, hanging valleys, valley-steps, and aretes ; 

 and in addition the evidence of former glaciers as represented by 

 moraines, rocbes moutonnees, and blocs perchees. 



2. There is sufficient evidence to show that the major land-forms 

 are pre-Glacial and that post-Glacial erosion is comparatively slight. 



3. Pre-Glacial erosion had sculptured a former upland plain into 



■' See Eamsay, Geology of North Wales (Mem. Geol. Surv.), p. 269 ; also 

 Quart. Journ. Geol. See, 1876, p. 116; also Greenly, Rep. Brit. Assoc. 

 Bradford, 1900, p. 737. 



