134 SOIL CONDITIONS AND PLANT GROWTH 



The vegetation areas correspond with the geological formations only 

 so long as the lithological characters remain constant. Some formations 

 aje very uniform, e.g. the Folkestone beds of the Lower Greensand, 

 but, in general, certain changes are observed. Where the formation has 

 been laid down in an estuary of no very great size, the coarser particles 

 are deposited near the old shore and the finer particles farther out, so 

 that a gradual change from finer to coarser soil is observed in travel- 

 ling along the formation, necessitating a soil division into two or three 

 vegetation areas ; the Hythe beds of the Lower Greensand and the 

 London clay afford illustrations. Considerable trouble often arises 

 when the formation consists of a number of strata of sands and clays, 

 and the outcrops are so narrow that no one type persists over any 

 large area. The simplest method of procedure is to map out any uni- 

 form areas that possess sufficient agricultural importance, and then 

 jjroup the remaining less important soils simply into gravels, sands, 

 loams, and clays. In dealing with drift soils, it is well first to map the 

 uniform areas and then look out for lines of uniformity and make up 

 j-egions within which the agricultural characteristics vary between a 

 higher and a lower limit. The investigator must be guided by the 

 importance of the region from the particular point of view in deciding 

 how closely he is to map out the country. 



Absolute uniformity cannot be expected over any considerable 

 area, and even such uniformity as existed has often been upset by sub- 

 sequent rearrangements of the soil which result in the original surface 

 being covered up with a later deposit or being washed away, leaving 

 the original subsoil to become the new soil. Such changes are readily 

 detected by mechanical analysis of the surface and subsoils ; examples 

 are given in Table LI I. At Merton the subsoil on the lower ground 

 appears to have been the original surface soil because of its identity 

 jvith the surface soil and its marked difference from the subsoil of the 

 land higher up ; it has been covered with a deposit identical with and 

 presumably derived from the higher lying soil. The same thing has 

 jiappened at Ramsey Green. At Woodchurch, however, it appears 

 that the old surface of 69, now the subsoil, has been covered with 

 rather a lighter soil ; it is equally possible, however, in this particular 

 case that 70 has lost its original surface soil, the present surface being 

 the bared subsoil. This kind of variation is common on clay soils and 

 often leads to differences in agricultural value that are fairly marked, 

 but not sufficient to affect the type. Normally the subsoil contains 

 more clay than the surface soil (as in no, 69 and 70) and any devia- 

 tion should be carefully investigated. 



