CHANGES OF STRATA 



223 



edges and to dovetail in with beds of a different character. When 

 the strata are of constant thickness for considerable distances, and 

 the bedding planes remain parallel, the stratification is said to be 

 regular. In many cases these changes take place rapidly from 



*,** 





"S^SSS^'^^ ^^i^t^^U* 



FlG. 79. — Cross-bedded sandstone. (Photograph by the Iowa Geological Survey.) 



point to point, and then the strata are plainly of lenticular shape, 

 thickest in the middle, thinning quickly to the edges. Here the 

 bedding planes are distinctly not parallel, and the stratification is 

 irregular. 



An example of rapid horizontal changes is given in the two 

 accompanying parallel sections (Fig. 78), taken through the same 



