272 



CONTEMPORANEOUS EROSION 



extends farther across the old land surface than the one beneath 

 it, and conceals the edges of the latter. The relation of overlap 

 is between the successive layers of a conformable series. 



Overlap may be a structure of much economic importance, 

 if one of the lower strata, say a coal-bed, is mined. It is not 

 safe to assume that wherever the upper beds of such a series are 

 found, the lower will be found directly beneath them, an assump- 

 tion which may result in costly failure. 



FIG. 119. — Contemporaneous erosion in limestone, Iowa. (Ph 

 Iowa Geological Survey.) 



bv the 



Contemporaneous Erosion. — It was stated above that the defi- 

 nition of unconformity, as given, would include certain structures, 

 which, nevertheless, must be distinguished from it : one of these 

 is contemporaneous erosion. This structure is produced when a 

 current of water excavates channels for itself in the still soft and 

 submerged mass of sediment. After the current has ceased to 

 flow, renewed deposition fills up the hollow with the same or a 



