yy ME. WM. SHONE OK THE STJBTEREANEAIf 



former line of junction of the clay with the sands and gravels^ 

 the latter must have been removed to the extent of the difference 

 between A and B, the present outcrop in each valley. In valley 

 No. 1 the clay has subsided 18 feet, and in valley No. 2 30 feet. 

 The total length of the section is only 480 yards. 



It is characteristic of subterranean erosion that its action is 

 lateral ; it is greatest at the point of escape and least at the farthest 

 distance from it, while the interval between these two extremes 

 becomes an inclined plane of subsidence. I believe this fact accounts 

 for the occurrence of the present outcrops of the sands and gravels 

 in the section at B-B instead of at Z-Z, the difference being the 

 measure of the subterranean erosion of the Drift. 



Subterranean erosion is frequently intermittent, and especially so 

 if an impervious stratum rests upon a pervious one. Thus the accom- 

 panying diagram (fig. 2) illustrates a very familiar example. The 



Pig. 2. 



X. Clay. 



Y. Sand and Gravel. 



Z. Eock. 



overlying clay, by the removal of the sands and gravels between 

 A and B, has brought down the clay'X upon the rock Z, effectually 

 stopping any further subterranean erosion until the barrier of clay 

 between A and B has been removed by subaerial denudation. 



The sands and gravels beneath the clay are not eroded in a uni- 

 form manner, the finer sands and gravels being most easily removed^ 

 The stratification of these beds is most variable, and current-bedding 

 very frequent, this giving rise to constantly changing deposits of 

 light sands intermingled with beds of coarse sands and gravels on 

 the same horizon. In sandpits it may be often observed that the 

 surface-soil is of very unequal depth, as in the following diagram- 

 matic section of Ithell's sandpit at Upton, Chester: — 



Fi?:. 3. 



A. Surface soil, 



B. Boulder Olay. 



C. Sand and Gravel. 



D. Banter Sandstone pebble 



