OLENTANGY SHALE AND DEVONIAN DEPOSITS 



487 



Southward from Kettle Point, Ontario, therefore, the Huron or 

 lower portion of the Ohio shale rests on older and older beds to 

 which its relationship must be that of unconformity (disconformity) . 

 This relation is not strikingly perceptible at any one place, but in 

 southern Ohio the time interval between the Silurian and Devonian 

 strata, which are in contact, is enormous. When it is recalled that 

 the first effect of running water on a newly uplifted land surface is 

 to roughen it, and that continued erosion tends to produce planeness, 

 it is clear that, where little or no folding or tilting of the stratified 

 rocks has taken place, slight (apparent) unconformities are likely to 



Fig. 6. — The sharp and slightly undulating contact between the Ohio and the 

 Olentangy shales in the clay pit at Delaware, Ohio. 



represent great intervals of time, while conspicuous ones may stand 

 for shorter intervals. Or, in other words, the greater the time inter- 

 val which is represented by an erosional unconformity (disconformity) 

 in undisturbed strata, the more evasive it is likely to be. This is 

 probably one of the chief reasons for the marked differences in the 

 interpretation of sections where such gaps in sedimentation occur. 

 With the Hamilton beds at Sandusky and in central Ohio resting on 

 the Delaware limestone (Lower Erian) and overlaid unconformably 

 by the Ohio shale in both places, the advisability of calling the soft 

 marly beds to the south of Sandusky, Olentangy shale seems to be 

 justified, even though the faunal evidence may not be as conclusive 

 as could be desired. 



