On the Agricultural Geology of England and Wales. 473 



From Wales we will now return to the mouths of the Mersey 

 and the Dee, and trace the erratic deposits thence into the interior, 

 over the red sandstone plain of Lancashire and Cheshire, up to 

 the very foot of the Penine chain, from Manchester to Congleton, 

 and southward towards Stafford. Shells have been found at each 

 of the two former places, and at various points in Shropshire and 

 the adjoining counties, the most southern yet known being about 

 three miles south of Worcester. Boulder clay occurs at greater 

 elevations in the interior of the island than on the coast. On the 

 Norfolk coast it is nowhere more than 100 feet above the level 

 of the sea ; but near Swaffham it attains an elevation of between 

 300 and 400, with a depth of 90 feet in some of the hollows in 

 the chalk. On the other hand, the upper sand and gravel, which 

 are nearly 300 feet thick at Trimingham Beacon, thin off on the 

 summit of the watershed at Swaffham, so as not to exceed a 

 thickness of 30 feet as the maximum. Similar differences occur 

 between the levels of the boulder clay on the coast of Cheshire 

 and in the interior, at the foot of the Penine chain. These dif- 

 ferent elevations may be explained by the advance of the coastline 

 into the interior during subsidence of the land ; the boulder clay 

 "being, as we have said, a littoral deposit. As the subsidence con- 



No. 1. 



Upper Erratics. 

 Lo^yer Erratics. 



tinued, this would be succeeded and covered by the upper erratic 

 deposits of a more open sea ; so that the lower erratics at a, in 

 the annexed diagram, would be contemporaneous with the upper 

 part of the upper erratics at l. As the subsidence proceeded, 

 and the climatal conditions favourable to the production of boulder 

 clay as a littoral deposit gave place to those favourable to the pro- 

 duction of sand and shingle, the ordinary deposits of such situa- 

 tions, these would ultimately overlap or extend beyond the 

 boulder clay at c. 



Simdar phenomena occur as the erratic tertiaries are traced 

 from north to south. They are thicker in the north than in the 

 interior of the country south of Trent, in which direction the 

 boulder clay appears to thin off. We have seen it between Fish- 

 guard and St. David's Head, southward of which our observa- 

 tions have not extended on the west of the island. East of the 

 chalk range it comes down to the neighbourhood of London, where 

 ff b\ the Hij^hj^ate ridge. We have observed it near Des- 

 borough, in Northamptonshire, where there can be no doubt that 

 the gravel beds described by the Dean of Llandaff. as extending 

 from Houghton-on-the-Hill,*^by Market^Harborough, to Daventry, 



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