12 THE ORIGIN OF SOILS [chap. 



effectively than by any artificial means. The fragments 

 that have been broken off the main rock will be con- 

 tinually reduced in size by successive frosts, until they 

 reach the ultimate fragments which are no longer 

 penetrated by water ; even in a soil the disintegration is 

 still proceeding. 



The weathering agencies just described would gradu- 

 ally cover any exposed rock with a layer of debris, 

 which would protect the lower layers from further action 

 were it not that the rain is always washing the finer 

 particles into the valleys and so leaving the rock open to 

 fresh attack. Even on grass land the fine mould brought 

 to the surface by worms, moles, ants, etc., is constantly 

 travelling downhill by the agency of rain. On arable 

 land containing stones it is a common expression to say 

 that the stones " grow " : however thoroughly the surface 

 may be picked clean of stones, in a year or two they will 

 seem as numerous as ever ; the fine soil gets washed 

 away to lower levels, leaving the stones standing upon 

 the surface. Even the stones themselves gradually creep 

 downhill, the rain undermines them till they fall over, 

 they must fall a little lower down the slope, until they 

 eventually reach the valley and are subject to further 

 transport by running water. At the bottom of many of 

 the smaller dry valleys on the chalk rests an enormous 

 accumulation of flints of all sizes ; in one case in a small 

 upland valley the deposit was 6 or 7 feet thick, and the 

 unworn flints were so close as to be practically in 

 contact, only the interstices being occupied by soil ; yet 

 the surface carried good crops. 



The material which thus creeps down the sides of 

 the valleys is further sorted out by the streams and 

 rivers and deposited as beds of gravel, sand, or clay, 

 the " alluvium " which underlies the level river meadows. 

 The coarser the material the more readily will it 



