470 Oil the Arjricultiiral Geology of England and JVales. 



is only partially exposed in the bottoms of the valleys. South 

 of Norwich a large area of the clay is uncovered, the two great 

 outliers of Strumpshaw and Foringland bearing witness to the 

 mass of upper erratics which has been removed. In the valley of 

 the Waveney, and through the centre of Suffolk, a larger area of 

 the boulder clay is exposed, constituting the strong land district 

 of Young's agricultural map, with numerous outliers of the gravel 

 and sand of the upper erratics remaining upon it. Similar phe- 

 nomena extend through Essex. The reconstruction of the 

 mixed materials of the two erratic deposits along certain lines — 

 their total removal from others, and the exposure of the red and 

 coralline crag, the London clay, and the chalk — together with the 

 varying depth and composition of the ""warp" or unconformable 

 superficial deposit — its depth varying with contours, and its com- 

 position with that of the beds exposed in the neighbourhood, 

 combine to produce the rapid and intricate changes of soil which, 

 in a former page, we have seen were attributed by Mr. Morton to 

 variations in the mineral character of beds of the plastic clay, but 

 which admit of no satisfactory explanation without reference to 

 the deposits of the erratic period. 



If these deposits are traced down our w"estern coast, cliffs of 

 varying height — composed in the lov/er part of boulder clay, and 

 in the upper part of sand and gravel — present themselves in the 

 tract between the Mersey and the Dee, and at various points 

 from the last-named river to Conwy,"^ particularly near Aber- 

 gele ; interrupted, however^ by promontories of lofty rock, on 

 which they could not have formed, and by low alluvial districts 

 from which they have been denuded. Similar accumulations, 

 with similar interruptions, are found along the coast of Anglesey, 

 from Red Wharf Bay to near Holyhead, and along both sides of 

 the long point of Caernarvonshire, from Clynog to the neighbour- 

 hood of Criccieih. 1 hey are found also at intervals along the 

 whole coast of South Wales, from Aberystwith to near St. David's 

 Head — the boulder clay chiefly occurring in patches at the 

 mouths of the small valleys of this rocky coast, and in the upper 

 parts of the large valleys, the lower regions of which appear to 

 have been swept clean of it. 



The sections afforded by the sea cliffs exhibit the structure of 

 the erratic tertiaries which fill depressions between the hills, and 

 constitute small basins analogous to the larger erratic districts on 

 the eastern side of the chalk range. The variations of soil from 

 sand to clay through all the intermediate gradations are caused 

 in the smaller, as well as in the larger basins, by the amount of 

 denudation to which the erratic strata have been subject, and by 



* Journ. Geol. Soc. Dubliu, vol. i. part iii. p. 1£0. 



