270 DRIFTS IN GREAT BRITAIN. 



^ S- Of the occurrence of such accumulations in Great Britain^ andoj 

 their influence in modifying the character of4he soil. 



Such accumulations, for example, present themselves over a large 

 portion of our own island. Thus, in Devonshire, the chalk and green 

 sand are so completely covered by gravels, consisting of the fragments 

 of older rocks from the higher grounds, mixed with chalk-flints and 

 chert, that nearly the whole of this tract possesses one common charac- 

 ter of infertility, and is widely covered with downs of furze and heath 

 (De La Beche.) In like manner the chalk, green sand, and plastic clay 

 of a large portion of Norfolk and Suffolk, and of parts of the counties of 

 Essex, Cambridge, Huntingdon, Bedford, Hertford, and Middlesex, are 

 covered with till, (stiff unstratified clay,) containing large stones, (boul- 

 ders,) or with gravels, in which are mixed fragments of rocks of various 

 ages, which must have been brought from great distances, and perhaps 

 from different directions (Lyell.) So over the great plain of the new 

 red sand-stone, in the centre and west of Eflgland — in Lancashire, 

 Cheshire, Shropshire, Staffordshire, and Worcestershire — drifted gra- 

 vels of various kinds are widely spread. It may indeed be generally 

 remarked, that over the bottoms of all our great vallies, such drifted 

 fragments are commonly diffused — that upon our wider plains, they are 

 here and there collected in great heaps — and that on the lower lands th-it 

 border either shore of our island, extensive deposits of clay, sand, or gra- 

 vel, not unfrequently cover to a great depth the subjacent rocks. 



The practical agriculturist will be able to confirm this remark, in 

 whatever district almost he may live, by facts which have come within 

 his own knowledge and observation. I shall briefly explain, by way of 

 illustration, the mode in which such accumulations of drifted matter 

 overlie the eastern or lower half of the county of Durham. 



The eastern half of the county of Durham reposes, to the north of the 

 city of Durham, chiefly upon the coal measures, (sand-stones and shales;) 

 to the south, chiefly on the magnesian lime-stone and the new-red sand- 

 stone. These coal measures rise, here and there, into considerable eleva- 

 tions, as at Gateshead Fell near Newcastle, and Brandon Hill near Dur- 

 ham, where the rocks lie immediately beneath the surface, and are cov- 

 ered by comparatively little transported matter. The magnesian lime- 

 stone, also, in many localities, starts up in the form of round hills or ridges, 

 on which reposes only a poor thin soil, formed in great measure by the 

 crumbling of the rock itself. Yet, generally speaking, this entire dis- 

 trict is overspread with a thick sheet of drifted matter, consisting of 

 clays, sands, and gravels. 



This drift is made up of three separate layers, to be observed more or 

 less distinctly in taking a general survey of the county, though there afe 

 few spots where they can all be seen reposing immediately one over the 

 other. 



1°. The upper layer consists of clays — on the higher grounds, poor, 

 stiff, yellow — on the hill-sides and slopes of the valleys, often darker in 

 colour — but almost everywhere full of rounded trap boulders* from a few 



* In some parts of Northumberland these trap boulders are still more numerous. In the 

 country which stretches between the north and south Tyne, the old grass fields are full of 

 them. A friend of mine informs me that in ploughing out a nine-acre field on his estate in 

 that district, there were dug out and carried off no less than 900 tots of such rolled stonea 

 freat and small ! 



