Soils 29 



Scattered along the existing rivers and around the southern edge of the 

 Fens, and constituting some of the "islands", are considerable areas of 

 Valley and River Gravels, of varying constitution. They give rise to soils 

 that are gravelly, brownish grey to grey-black in colour, and loamy sands 

 to medium loams in texture. The soils are free draining, but many of them, 

 through their position, have high water tables except in so far as these are 

 lowered by field drainage. Much of the material of the Gravel deposits is 

 calcareous, but leaching has reduced the percentage of calcium carbonate 

 in the top soils to very low values. The ground water is calcareous, 

 however, and seriously acid soils are rare. 



The Fen Alluvial deposits cover about half the area of Cambridgeshire. 

 The soils which are characteristic of Fenland are of four main kinds — peat, 

 silt, shell marl and skirt. All these soils have been formed from materials 

 laid down in association with a river system containing calcareous waters, 

 a fact which has had an important result on the fertility of the soils ; they 

 are almost all rich in calcium in one form or another. The surface material, 

 of varying constitution, is the result of the deposition of inorganic particles 

 by the rivers and estuaries in their meanderings and frequent floodings, 

 and of inorganic material by the growth of vegetation in swamp and 

 marginal conditions. The variation of these main factors from time to 

 time, and from point to point, has resulted (a) in a "profile" of great 

 complexity at any point, and {b) in considerable variations in existing soil 

 conditions from place to place. 



The Peat soils are composed mostly of organic matter derived from 

 swamp vegetation. They are black or nearly so, light, spongy, and 

 crumbly. The organic matter contains considerable amounts of exchange- 

 able calcium and is frequently associated with a small amount of free 

 calcium carbonate. The depth of the peat deposit varies considerably from 

 place to place, from a few inches to 10-15 ft-! but it is not always 

 in an uninterrupted layer. Occasionally, as many as five separate 

 bands have been proved, interlayered with "buttery clay", silt, or sandy 

 deposits. Much of the peat does not lie directly on the older formations 

 but on the characteristic clay, a dark blue-grey greasy material, entirely 

 unlike the older clay formations. The depth of the peat' and the nature of 

 the underlying material are points of major importance in determining 

 the agricultural value of the land.' 



The Silt soils are associated chiefly with areas of marine deposits of fine 

 rounded quartz grains with small flakes of muscovite, occurring at the 

 seaward or northern end of the County. They show a big range of texture 



' For the shrinkage and wastage of the peat, see p. 186 below. 



^ For the practice of "claying" the peat, see pp. 120-1 and 152 below. 



