14 CLAYS AND CLAY INDUSTRY. 



SECONDARY CHANGES IN CLAY DEPOSITS. 



Changes often take place in clays subsequent to their deposition. 

 These may be local or widespread, and in many cases either 

 greatly improve the deposit or render it worthless. The marked 

 effect of some of these changes is often well seen in some clay 

 beds of which only a portion has been altered. These secondary 

 changes are of two kinds, viz., mechanical and chemical. 



Mechanical Changes. 



Tilting, Folding, Faulting. — From the above explanation of 

 the origin of sedimentary clays, it has been seen that they were 

 laid down under water. Since they now appear at the surface, 

 the water has been drained off, either by the water level falling, 

 or the sea bottom being elevated. In the latter case, since the 

 elevation of a large area is rarely the same at all points, a tilting 

 of the beds may occur during the uplift. This is the case with 

 all the beds of. clay and sand in South Jersey, all of which now 

 tilt or dip gently to the southeast, although they were essentially 

 horizontal when deposited. Beds of clay and shale sometimes 

 shew folds or undulations. In the case of consolidated or hard 

 beds, these may be due to lateral pressure, caused by movements 

 in the earth's crust, while in soft beds the cause is frequently 

 local. Thus, in the clay beds about Woodbridge, instances of 

 local folding were noted where the disturbance seems to have 

 been due to the shoving action of the ice sheet during the Glacial 

 period. Such folds, however, are of minor account and affect 

 only a few beds. 



Where beds of clay are gently folded into arches (anticlinal 

 folds) and troughs (synclinal folds), each bed slopes or dips 

 away from the axis of an anticlinal fold and towards the axis 

 of a synclinal fold, but if followed parallel to the axis it will 

 remain at the same level, provided the axis itself is horizontal. 

 Since, however, the clay strata, of New Jersey are rarely folded, 

 these considerations are not important here. 



