On the Agricultural Geology of England and Wales. 469 



erratics maj be attributed to the decrease, daring the formation of 

 the latter, of neighbouring land as submergence proceeded, and in 

 part to the mitigated rigour of the cbmate. 



Both deposits contain marine shells, more abundantly in some 

 districts than in others, In both the shells are much broken, and 

 there is an intermixture of species of different habits. In both 

 regular beds of them are extremely rare. All beds of this kind 

 which we have ourselves examined appear referable either to the 

 period which immediately preceded the commencement of the 

 erratic phenomena, or to that which characterised its close, and 

 the passage into the alluvial period. With respect to the height 

 to which these deposits ascend, all that can be determined, in 

 districts like Norfolk, whose elevation is less than 600 feet, is, 

 that their highest tracts were submerged. Regions of greater 

 elevation, like the Cambrian, Cumbrian, and Penine chains, af- 

 forded evidence of submergence to a much greater depth. 



" It is marvellous," says Mr. Darwin, " that Nature should 

 have marked, by buoys made of stone, the former sinking of the 

 earth's crust, and likewise, I may add, its subsequent elevation ; 

 and that on these blocks of stone the temperature during the long 

 period of their transportal may be said to be plainly engraved."* 



On the Penine chain erratic blocks are found, as we have stated, 

 at the height of 1400 feet ; and there is reason to believe that the 

 Cumbrian chain was submerged to about the ^ame depth. 



On Moel Tryfan, in Caernarvonshire, shells and granitic 

 detritus, with chalk flints, have been found at somewhat less than 

 1392 feet. Blocks of granite are lodged at heights of about 

 1000 feet on the northern flanks of the Welch mountains. Shells 

 and blocks and pebbles of granite are spread over the new red 

 sandstone and coal-measures of Lancashire and Cheshire, up to 

 600 or 700 feet. Granitic detritus is lodged in various parts of 

 the Penine chain at much greater elevations, not exceeding, how- 

 ever, 1400 feet. These deposits are seen in their most perfect 

 state in low-lying districts, where sections are exhibited in the 

 sea cliffs. If we trace them down the eastern coast we find them 

 in Holderness to consist of boulder clay or till, from which the 

 upper erratics have been stripped, with the exception of a few 

 detached outliers of sand and gravel. In the cliffs of Cromer 

 we have the boulder clay at the base, varying in thickness 

 from ten to somewhat less than one hundred feet. The original 

 thickness of the upper erratics there amounts, in their greatest 

 developement, to nearly 300 feet. In the district north of 

 Norwich, they have been so slightly denuded that the clay 



"On tiie Transportal of Erratic Boulders."^ — Quart. Journ. Geol. Soc, vol. iv. 

 p. 315 (1848). 



