SOIL : ITS ORIGIN AND NATURE. I3I 



19. Soil: its Origin and Nature. 



By Professor James Geikie. LL.D., D.C.L., F.R.S. 

 ( Concluded from p. 21.) 



III. 

 {b) Drift Soils. 



These are soils which do not arise from the disintegration 

 of immediately underlying bed-rock, but are simply the modified 

 upper portions of superficial formations of every kind. The 

 materials of which they are composed have been transported 

 for shorter or longer distances. 



Glacial Soils. — -These soils overlie superficial accumulations 

 of glacial origin — the more typical of which are certain kinds 

 of clay. 



Till or Boulder- Clay. — This clay is usually tough and 

 tenacious, more or less highly impermeable, and abundantly 

 crowded with stones and boulders of all shapes and sizes. 

 From an agricultural point of view its most notable character 

 is not so much its impermeability as its composition. It con- 

 sists almost exclusively of " unweathered " rock-material — its 

 mineral constituents are quite unaltered, and it is, therefore, 

 in no sense of the term a " subsoil." It plays the part, in fact, 

 of unweathered bed-rock. It appears to have originated under 

 the great ice-sheet which formerly covered our country, and 

 which, flowing off" in various directions, crushed, ground, and 

 pulverised the underlying bed-rocks. Its formation, therefore, 

 was entirely mechanical — the chemical action of the atmosphere 

 and rain being excluded. The " clay " consists, to a very large 

 extent, of a fine rock-meal or flour, which, although it may 

 contain all the elements met with in ordinary fertile soils, is 

 nevertheless barren, since the constituent mineral matter is not 

 in a fit condition to be assimilated by plants. Owing to the 

 general impermeable character of boulder-clay, a soil-cap forms 

 very slowly upon it. But all tills are not alike in this respect. 

 In some parts of the country, where till consists largely of 

 crushed sandstone, it is somewhat more permeable, and yields 

 a thicker soil-cap, the upper part of which, a foot or less in 

 thickness, forms a strong loam or loamy clay-soil. Not only 

 the composition but the colour of the till varies. This is readily 



