82 THE FERTILITY OF THE SOIL [CH. 



lumps like stones and shatters into little fragments 

 under sun and frost They use the sandy kind if no 

 other is available, and in any case on marshy soil. 

 The Ubii are the only people I know of who make 

 land fertile by digging up the soil to a depth of about 

 3 feet and then throwing on top a foot's thickness. 

 That kind does not last more than 10 years." 



To this day the "silvery chalk" is dug out in 

 Hertfordshire just as Pliny describes : a well is sunk 

 and widened out to a chamber as the chalk is reached: 

 the chalk is hauled up and spread on the ground. 

 The well is partly filled in and leaves one of those 

 dells so characteristic of the fields of the county. 



The method that Pliny ascribes to the Ubii has 

 been very much used. Light barren sands are often 

 underlain by heavier loam or clay which when brought 

 to the surface give rise to a fertile soil. Instances 

 will be given in Chap. VII. 



It is not always necessary that the material should 

 be carried by human labour; the forces of Nature 

 can sometimes be utilised. Perhaps the best illustra- 

 tion is furnished by warping, a process introduced 

 during the early eighteenth century into North Lin- 

 colnshire and South East Yorkshire and practised 

 under the name of Colmatage in Tuscany, Romagna, 

 and the neighbourhood of Naples. 



The Ouse, the Trent and other rivers connected 

 with the Humber are tidal, and as the flood travels 



