158 GLACIATION OF WALES. [Ch. XII. 



part of an estuary overflowed at high tide into dry land, they had 

 thrown into it a vast load of stones and sand, upwards of 900,000 

 cubic yards in volume. Under this weight the mud had sunk down 

 many yards vertically. Meanwhile the adjoining bottom of the estu- 

 ary, supporting a dense growth of salt-water plants, only visible at low 

 tide, had been pushed gradually upward, in the course of many 

 months, so as to project five or six feet above high-water mark. The 

 upraised mass was bent into five or six anticlinal folds, and below the 

 upper layer of turf, consisting of salt-marsh plants, mud was seen 

 above the level of high tide, full of sea-shells, such as Mya arenaria, 

 Modiola plicatula, Sdnguinolaria fusca, Nassa obsoleta, Naiica triseri- 

 ata, and others. In some of these curved beds the layers of shells 

 were quite vertical. The upraised area was 75 feet wide, and several 

 hundred yards long. Were an equal load, melted out of icebergs or 

 coast-ice, thrown down on the floor of a sea, consisting of soft mud 

 and sand, similar disturbances and contortions might result in some 

 adjacent pliant strata, yet the underlying more solid rocks might re- 

 main undisturbed, and newer formations, perfectly horizontal, might 

 be afterwards superimposed. 



Glaciation of Wales, England, and Ireland. — The mountains of 

 North Wales were recognized, in 1842, by Dr. Buckland, as having 

 been an independent centre of the dispersion of erratics, — great 

 glaciers, long since extinct, having radiated from the Snowdonian 

 heights in Carnarvonshire, through seven principal valleys toward as 

 many points of the compass, carrying with them large stony fragments, 

 and grooving the subjacent rocks in as many directions. 



Besides this evidence of land glaciers, Mr. Trimmer had previously, 

 in 1831, detected the signs of a great submergence in Wales in the 

 Post-pliocene period. He had observed stratified drift, from which 

 he obtained about a dozen species of marine shells, near the summit 

 of Moel Tryfaen, a hill 1400 feet high, on the south side of the 

 Menai Straits. Although his observations were afterwards confirmed 

 by the late E. Forbes, and still later by Mr. Prestwich and Professor 

 Ramsay, doubts as to the nature and age of the deposit still lingered 

 in many minds. But on these subjects all doubt has at length been 

 removed by aid of a long and deep cutting made through the drift in 

 1863 by the Alexandra Mining Company in search of slates. In this 

 cutting a stratified mass of incoherent sand and gravel, 35 feet thick, 

 was laid open near the summit of Moel Tryfaen, containing shells, 

 some entire, but most of them in fragments. In the summer of 1863 

 I examined the newly-opened section in company with the Rev. W. S. 

 Symonds, and we obtained 20 species of shells on the spot, and found 

 in the lowest beds of the drift large heavy boulders of far-transported 

 rocks, glacially polished and scratched on more than one side. Un- 

 derneath the whole we saw the edges of vertical slates exposed to 

 view, which here, like the rocks in other parts of Wales, some at 

 greater and some at less elevations, exhibit, beneath the drift, unequi- 



