1867.] Ice Marks in North Wales. 35 



rating briefly what has been observed elsewhere, and adding some 

 account of the more interesting problems and discussions to which 

 they have given rise. 



1st. The Glacial or Northern Drift. — This is a layer of 

 loose materials — gravel, clay, mud, pebbles, and angular stones — 

 which is found spread at intervals all over Northern Europe, and is 

 very common in the valleys and upland slopes of North Wales. It 

 is very abundant all round the town of Dolgelly, where it forms 

 undulating slopes, mounds, and hummocks in most of the valleys, 

 filling up the space between the flat alluvial meadows on the river 

 side, and the steep rocky slopes of the adjacent mountains. Wher- 

 ever this is cut through in making roads or railways, it is seen to 

 be full of blocks of stone, pebbles, and large masses of rock, dis- 

 tributed through it without any order or arrangement, the top, 

 middle, and bottom being alike in composition. From the contour 

 of the surrounding mountains it can be often seen that this deposit 

 is of great though very variable thickness, probably often exceeding 

 a hundred feet, and it certainly covers many hundred square miles 

 of country in North Wales alone. On ascending the mountains it 

 is often found on their less precipitous slopes and in the upland 

 valleys, at more than a thousand feet elevation ; and it has even 

 been traced around Snowdon by Professor Eamsay to a height of 

 more than two thousand feet. The materials of which the drift is 

 composed are various. Sometimes the rocks are nearly all those of 

 the surrounding mountains, at other times they are such as must 

 have been brought from a great distance. The geological age of 

 the drift is determined by its overlying all, even the most recent 

 formations, and by its containing occasionally marine shells of an 

 arctic type and of species which are all now living. 



Here we have materials of a loose and miscellaneous nature 

 which were deposited in the sea but not by the sea. That the 

 drift was deposited in the sea is proved by the marine shells which 

 have been found in it up to the height of 1,300 feet on some 

 mountains of Carnarvonshire ; and we have thus a proof that North 

 Wales was at a very recent epoch sunk to at least that depth 

 beneath the ocean. The presence of the drift itself, however, at a 

 height of more than 2,000 feet, would prove a much greater sub- 

 mergence. That the deposit could not have been made by the sea, 

 is shown by the want of arrangement of the materials and the 

 abundance of large angular fragments of rock. Water always sorts 

 the materials it deposits. The rocks, the pebbles, the shingle, the 

 sand, the mud, are carried different distances, and deposited in 

 different places or in different layers. Water deposits are stratified. 

 Neither can rocks be carried far by water and retain their angles 

 and clean fractured surfaces. They get rounded into boulders or 



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