208 GEOLOGY OF THE ISLE OF WIGHT. 



CHAPTER XIII. 



PLEISTOCENE AND BECENT DEPOSITS. 

 Classification. 



The boundaries of these deposits have now for the first time 

 been drawn on the one-inch map of the Isle of Wight, In the 

 course of this examination some problems of great interest in 

 connection with the physical history of the Island have been 

 opened, up ; among them the question of the relative age of the 

 older crravels of the south of England and of the Glacial Deposits, 

 the age of the river valleys, and the date of the separation of the 

 Island from the main land. 



The classification of the superficial deposits presents consider- 

 able difficulty, for though the gravels of different areas indicate a 

 similar sequence of events, yet the events in any two areas may 

 not have been contemporaneous. The period, moreover, during 

 which the gravels have been forming, though undoubtedly pro- 

 lono-ed, does not seem to have been broken up by any marked 

 chano"es of physical conditions, so that no classification can be 

 proposed in which the deposits of one group shall not overlap in 

 time those of another. Yet the position and character of the 

 oldest gravel bring before us a picture of physical conditions so 

 entirely different to those of the present day, that some classifi- 

 cation by age becomes necessaiy. 



In the first place, an important series of gravels occurs near and 

 often on the watersheds by which the existing valleys of the 

 Island are divided, and forms well-marked plateaus. Though we 

 have no o-uide as to the relative age of the separate patches of 

 these o-ravels, except the doubtful test of height above the sea, 

 yet the similarity in their mode of occurrence justifies their being 

 o-rouped together under the title of Plateau Gravels. These 

 o-ravels were obviously laid down before the valleys in their 

 present form had been excavated. Yet their distribution and the 

 direction of the slopes on which they rest point to a drainage 

 system bearing some relation to that which now exists. 



A second group of gravels is arranged as terraces along the 

 sides and lower parts of the valleys, and though, like the 

 Plateau Gravels, now undergoing removal by the modern streams, 

 yet showing an obvious connection with their valleys. 



Lastly, come the alluvial and peaty deposits still in process of 

 formation along the courses of the streams, or such as might have 

 been formed by the existing streams. 



Three principal groups may thus be established in the Super- 

 ficial Deposits, capable of being arranged in chronological 

 order. But other deposits of importance occur which cannot be 



