TKACES OF A POST-GLACIAL FLOOD IN COENWALL. 241 



showing that such, a flood has never been repeated in our 

 County. 



The upper and basin-like portions of our valleys, — as 

 Tregoss Moors, are filled with drift gravel ; and similar gravel 

 is found in contact with the underlying rock throughout these 

 valleys to the sea. 



The mass of drift gravel on the high hill-top near 

 St. Kevern, may be traced by its trail down the slope of the hiU 

 to the sea at the Lowlands : and a gravel bed on the Crest of 

 the Hoe at Plymouth may in like manner be traced southward 

 along the surface to the so-called raised Beach at the sea shore. 



That the sea has in Cornwall overwhelmed the land to at 

 least 100 feet above its present level is now conclusively proved, 

 by the discovery of a large quantity of sea shells imbedded in 

 what I considered to be boulder clay at about 100 feet above the 

 mean level of the sea at St. Erth Grlebe, and described in my 

 paper in the Transactions of the Eoyal Geological Society of 

 Cornwall, 1881. These shells have been more fully examined 

 and described by Mr. Searles Wood, F.G.S., in a paper read 

 before the Geological Society of London, the 5th of November, 

 1884. A most careful examination of these shells by Mr. Searles 

 Wood, Dr. Gwyn Jeffreys, and Mr. Eobert Bell, had led to the 

 provisional conclusions that they belong to a pre-glacial deposit, 

 — that of the Red Crag. 



But whatever may be the geological age of these beds of 

 Sand and Clay at St. Erth, the marine shells which they contain 

 in the interstratified clay, demonstrate the oscillation to which 

 the surface of our County has been subject in recent geological 

 time. 



On the other hand there is geological proof from the 

 uniformity of the sections of our valley deposits, that the violent 

 flood of water which swept over the land depositing the stream 

 tin at the base of the vallej^s, and nothing but ordinary river 

 alluvium above, has never been repeated by any subsequent 

 flood ; and that these valley deposits have never been disturbed 

 by glacial action. 



Thus we are di-iven to the conclusion that the flood which 

 deposited the stream tin, was post-glacial ; and therefore, 

 probably the equivalent of that "great post-glacial flood," of 



