116 A. C. RAMSAY ON THE ISLAND OF ANGLESEY. 



14. How Anglesey became an Island. By Professor A. C. Ramsay, 

 LLi)., F.R.S., V.P.GLS. (Read January 19, 1876.) 



[Plate XIV.] 



The structure of the low ground of Caernarvonshire within three or 

 four miles of the Menai Straits in almost all respects resembles that 

 of Anglesey, both in its geology and physical geography. The 

 Menai Straits divide the two regions ; but carboniferous rocks form 

 the larger part of either shore, and the Straits may be considered 

 simply as a long shallow valley, the bottom of which happens to lie 

 beneath the level cf the sea. The question thus arises, At what 

 epoch and by what means was Anglesey separated from the main- 

 land? 



The whole of Anglesey is low ; and only one steep escarpment, a 

 minor one, occurs in the island — that of the Old Red Sandstone 

 overlooking Traeth Dulas, which rises abruptly above the tidal fiat 

 of the Traeth to the height of about 250 feet. (See Map and 

 Section PI. XIV.). 



The entire island may, indeed, be looked en as a gently undulating 

 plain, the higher parts of which attain an average elevation of from 

 200 to 300 feet above the level of the sea ; while most of its principal 

 brooks and small rivers run north-east and south-west, in depres- 

 sions with gently sloping sides ; and only one inland valley, with the 

 same trend, is of any marked importance, namely that of Malldraeth 

 Marsh, in which the small coalfield lies. There are, however, a few 

 exceptions tc the average levels mentioned above — the summit of Holy- 

 head mountain being 709 feet, and uraro, near Llanfairynghornwy, 

 558 feet above the sea, while the greatest elevation crossed by the 

 sections of the Geological Survey (sheet Xo. 40) is only about 

 400 feet high. 



On the opposite side cf the Straits the same kind of low undu- 

 lating scenery prevails for several miles inland, with the same kind 

 of minor north-east valleys, one marked instance of which occurs in 

 a long shallow and narrow valley in or alongside cf which the Caer- 

 narvon and Bangor road runs for several miles. 



The surface of the ground on both sides of the Straits is to a con- 

 siderable extent composed cf glacial detritus, with erratic boulders, 

 large and small (from the north), gravel, sometimes sand, and clay 

 from which any number of ice-scratched stones may be gathered 

 from well-exposed sections, as, for example, in the boulder-clay coast 

 cliff of the Mount at Beaumaris (which contains minute fragments 

 of sea-shells), or anywhere else in similar cliffs round the shores of 

 Anglesey, or inland in occasional pits and fresh cuttings on both 

 sides of the Straits. Through these glacial accumulations the rocks 

 of the country frequently appear, sometimes in barren tracts of 

 considerable extent, sometimes in small isolated bosses of gneiss or 

 grit, often covered with heath or furze, while the more fertile grounds 



