40 



Correspondence — Mr, A. H. Green, 



water-courses, and following in the footsteps of their pre-glacial 

 pioneers. 



With these facts before me I see no escape from the inference, 

 that, in this district at least, the Glacial Sea, so far from forming the 

 " primary " valleys, in which it left the Boulder-clay, had not even 

 the power to obliterate, in those valleys, many minor features formed 

 by previous subaerial action. E. H. Tiddeman. 



Geological Survey of Great Britain, 

 Clitheroe, December I2th, 1867. 



SEA-CLIFFS AND ESCARPMENTS. 

 Sir, — In combating the notion that escarpments have been origin- 

 ally sea-cliffs, Mr. Whitaker has stated so fully and forcibly the 

 well-known fact that their bases are rarely or never at the same 

 height above the sea-level for any distance, that there would at 

 first sight seem to be little room for anything more on the subject. 

 Mr. Whitaker's observations, however, having been confined to the 

 Tertiary and Secondary rocks of the South-East of England, it may 

 perhaps be well to shew that his remarks apply equally to the es- 

 carpments of other districts. I also note that one of your corres- 

 pondents still holds that depression would convert most of the prin- 

 cipal escarpments of the Centre and North of England into sea-cliffs ; 

 and another, while he admits that the bases of escarpments are not 

 strictly horizontal, seems to think that their deviations from a level 

 line are either so small, or so very gradual, that they might be con- 

 verted into sea-cliffs by inequalities of depression by no means be- 

 yond the limits of probabilit3\ Vague statements, like the latter, are 

 easily made; but, before they can carry any weight with them, they 

 must undergo the test of facts, and figures ; and for this end I have 

 drawn up the following table shewing the details of two cases. 

 Escarpment surrounding the Rivelin Valley near Sheffield. 



NORTH SIDE OF 



VAT.T.KY. 



SOUTH 



SIDE OF THE VALLEY. 1 



Height of 



Distance 



DiflFerence be- 



Height of 



Distance 



Difference be- 



from start- 



tween height 



from start- 



tween height 





ing point 



of base at each 



sea-level 

 in feet. 



ing point 



of base at each 



in feet. 



in miles 



point and mean 



in miles 



point and mean 



and chains. 



height of base. 



and chains. 



height of base. 



1000 





+ 300 



1000 





+ 300 



1075 



0-40 



+ 375 



1075 



0-42 



+ 375 



1000 



0-60 



+ 300 



1000 



0-76 



+ 300 



900 



1- 



+ 200 



900 



1-35 



+ 200 



800 



M5 



+ 100 



800 



1-72 



+ 100 i. 

 + 100) ~ 



775 



2- 



. .}. 



800 



3-50 



700 



2-45 



700 



3-65 





600 



2-48 



— 100 



600 



4- 6 



— 100 



600 



3-40 



— 200 



500 



4-18 



— 200 



400 



3-60 



— 300 



400 



4-25 



— 300 



325 



4- 



— 375 



326 



435 



— 376 



Let us now see what will be required to make this line a sea-cliff, 

 or, if it were once a sea-cliff, to convert it into the present escarpment. 



• Fault between these points. 



-t- Between these points the base|;is up and down between 800 and 850. 



