12 THE ORIGIN OF SOILS [chap. 



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 

 settle, the finer particles are only deposited when the 

 velocity of the stream has been almost entirely checked. 

 The gravel and sand are deposited in and about the 

 stream course itself, the finer material falls on the meadows 

 in flood time, so that their level is gradually raised from 

 year to year. Wherever the meadows get water-logged 

 the surface vegetation will begin to accumulate as 



