28o PHYSIOGRAPHY. [chap. 



then cut its way down to the lower level, No 2. The higher 

 deposit is consequently the older, and is distinguished some- 

 times as the high-level gravel ; while the lower deposit, which 

 is of more recent age, is known as the lo7ci-level or valley 

 gravel. In a section of strata, such as that in Cannon 

 Street, the uppermost beds are the most recent ; but, on 

 the banks of a river, the higher deposits are presumably 

 the oldest, and will contain the remains of animals that 

 inhabited the valley before the river reached its lower level. ^ 

 In many places around London, the sheet of gravel is 

 overlaid by a thin deposit of brownish loam^ represented on 



Fig. 82. — Section across the Valley of the Wa.idle, shjwing high-level gravels and 



valley-jjravei. 



the map as brick-earth, since it is largely worked by brick- 

 makers. This earth forms an e.xcellent soil for vegetables, 

 and many of the market-gardens at Fulham and elsewhere 

 are situated upon it. It is probable that this brick-earth 

 has been thrown down by the river in flood. When the 

 Thames has overflowed its banks, it has deposited silt upon 

 the neighbouring land ; or, possibly, there may have been a 



1 There is no real contradiction here. Tlie higher gravels of tlie 

 river valley do not lie upon, but only at a higher level than, the lower 

 gravels. There are no exceptions to the rule, that of two strata, one 

 superimposed on the other, deposited by water action, the upper is the 

 more recent, if the beds have not been disturbed since their deposition. 



- Loam is a sandy clay ; marl a calcareous clay. 



