224 



THE STRUCTURE OF STRATIFIED ROCKS 



beds, only twenty feet apart. In these sections the differences of 

 thickness of the coal seams and of the sands and clays which 

 separate them are very striking. 



The finer details of structure of the stratified rocks likewise 

 afford valuable testimony as to the circumstances under which the 

 rocks were laid down. 



Cross-bedding (also called false or current bedding) is pro- 



Fig. 



Ripple marks on a modern sea-beach. (U. S. G. S.) 



duced by a strong current pushing sediment along the bottom and 

 thus bringing about an oblique lamination, or by the plunge of a 

 wave piling up material in heaps. Cross-bedded layers frequently 

 alternate with horizontally bedded ones, formed in slack water or 

 at ebb tide. Cross-bedding is most common in sandstones, but 

 it may occur in other rocks, even in limestones, though the latter 

 are comparatively seldom accumulated in such shallow water. 



