On the Agricultural Geology of England and Wales. 471 



the varying depth and composition of the unconformable deposit, 

 or warp, spread over the denuded surface. 



The lower erratics of mountainous regions do not extend to 

 heights which we should estimate at more than 700 or 800 feet. 

 At greater elevations, the gravel and sand of the upper erratics, 

 which, at low elevations, rest on boulder clay, are in imme- 

 diate contact with the rock. As the clay ceases at a certain 

 height, so, at an elevation, which we estimate at somewhat more 

 than 1500 feet, the upper erratics also cease, and are replaced by 

 peaks of naked rock, surrounded by heaps of their own ruins in 

 the form of angular blocks. As the phenomena at such elevations 

 have a geological rather than an agricultural interest, we pass them 

 over without further notice than that they are in unison with the 

 recorded action of a polar climate on the land. The marine 

 remains, and the far transported boulders which surround the 

 Welch mountains, and are lodged on their flanks at considerable 

 elevations, have not penetrated far into the interior ; nevertheless 

 there are, even in the inland valleys, considerable accumulations 

 both of clay, gravel, and boulders, which have a local, or a modi- 

 fied local origin, having been derived from the neighbouring 

 rocks, and exhibiting in most cases proofs of som^e degree of 

 transportation outwards, along existing lines of drainage, but 

 greatly above their present influence. 



The most remarkable case which we have met with of an ad- 

 vance of the marine erratic deposits into the interior is in a small 

 valley, at an elevation of something less than 1000 feet, trans- 

 verse to the great valley of Nant Francon, and opposite to the 

 Penrhyn slate quarry, in Caernarvonshire. We there saw frag- 

 ments of granite and carboniferous limestone, accompanied, as we 

 have good reason to believe, by marine shells, brought up from 

 the bottom of a shaft sunk through detrital deposits to the depth 

 of 60 feet ; the rock below being clay slate, a large portion of the 

 upper part consisting of local debris. In the valley of Nant 

 Francon, at lower levels, the detritus is wholly local, derived 

 from the mountains of the interior, and the rocks have in many 

 places that rounded, polished, and grooved surface, which is an 

 observed effect of the action of shore-ice, and also of terrestrial 

 glaciers. The marine deposit which fills the bottom of this 

 elevated transverse valley could only have reached it through 

 the main valley of Nant Francon, which communicates with the 

 sea ; and as they are not found in that valley, they must have been 

 sw^ept out of it, either by ordinary denuding action during the 

 process of elevation, or, as w'as suggested by Mr. Darwin, by 

 the descent of glaciers after the emergence of the land. The 

 polished and striated rocks may, therefore, be referred to the 

 action of such glaciers, or they may be considered m-onuments 



