V ] DRIFT SOILS 83 



tides and currents scour it free. These deposits made 

 by the river give us a clue to the origin of that other 

 arrangement of rock and soil that we have noted as 

 sometimes prevailing, when a clean surface of the 

 underlying rock is covered by gravel or sand or clay of a 

 totally different character, without any of the gradual 

 transition that marks the passage of a rock into a 

 sedentary soil. The layer which thus rests upon a rock 

 out of which it has obviously not been formed is never 

 solid rock, but is an assemblage of divided material that 

 is generally pretty uniform in character, wholly gravel 

 or wholly sand, or entirely made up of a uniform silt 

 like a brick earth. The uniformity is due to the fact 

 that the material has been transported to its present 

 position by running water and has undergone a sorting 

 in the process. Gravels must have been laid down in 

 the bed of the stream, even though the stream no 

 longer runs in that spot, but perhaps in a course at a 

 lower level in the valley. In such cases the gravel is 

 generally the remains of a much larger tract of gravel 

 which filled the valley to a higher level at a time when 

 the stream ran in far greater volume. Similarly the 

 sand must also have been deposited where the current 

 still ran pretty sharply, whereas the fine-grained and 

 uniform silts, such as constitute the brick earth patches, 

 were either deposited in quiet backwaters and laybyes, 

 or were dropped by slow degrees on the river meadows 

 in flood time, just as similar stuff is accumulating to-day 

 in the same sort of places. 



In addition to these non-sedentary soils that have 

 been moved and redeposited by running water or 

 by wind, over considerable parts of the country, 

 especially in the north and the midlands of Great 

 Britain, and for a hundred miles or more south of 

 the Great Lakes in the United States, we find the 



