26 



CLAYS AND CLAY INDUSTRY. 



for the sides of such excavations wash down rapidly, and a 

 muddy-red surface clay or loam will often run down over a bed 

 of lighter colored clay beneath SO' as to completely hide it from' 

 view. If the cut is deep and freshly made, the depth of weather- 

 ing can frequently be determined. 



Springs. — In many cases the presence of clay is shown by the 

 occurrence of one or more springs issuing from the same level 

 along some hill slope. These are caused by surface waters seep- 

 ing down from the surface (Fig. 18), until they reach the top 



Fig. 18. 



Formation of spring due to ground water following a clay layer. 



of some impervious clay stratum, which they then follow to the 

 face of the bank where they issue. The presence of springs, 

 however, cannot be used as a positive indication of clay, for a bed 

 of cemented iron sand, or even dense silt, may produce the same 

 effect (Fig. 19). 



5 



>pnng 



' y^^B^0!^^M$^:- 



loose 

 sand 



Cemented 

 sand 



Fig. 19. 



Formation of a spring due to a layer of cemented sand. 



Ponds. — In many regions covered by glacial drift, pools of 

 water are often retained in depressions because of the presence 



