ox THE DRIFTS OF THE THAMES VALLEY, ETC, 155 



10. On ^7i6 Relation 0/ ^7i<? Westleton Shingle to o^/^er Pre-Gla.cial 

 Drifts in the Thames Basin, and on a Southern Drift, ivith 

 Observations on the Final Elevation and Initial Subaerial 

 Denudation of the Weald : and on the Genesis of the Thames. 

 By Joseph Prestwich, D.C.L., P.B.S., &c. (Read Pebruary 26, 

 1890.) 



PART III. 



The Southern Drift, 



[Plate YIII.] 



Page 



1. Distinctive Characters of the Southern Drift 155 



2. Its Distribution in Kent, Surrey, Hampshire, Berkshire, 



Wiltshire 156 



3. Other Pre-Grlacial Hill-Gravels : the Warley and Brentwood 



Groups 162 



4. Early Physiographical Conditions of the Wealden Area 166 



5. Origin of the Southern Drift 168 



6. Relation of the Southern Drift to the Westleton Shingle and 



other Pre-G-lacial Drifts 174 



7. Main Lines of Elevation and Drainage of the South-east of 



England; Genesis of the Thames 176 



8. General Summary 178 



1. Distinctive Characters of the Southern Drift. 



Takingt the strata in their order of succession, this paper ought to 

 have preceded the last, but for the fact that for the purpose of 

 classification it was essential to have a base-line of which the posi- 

 tion had been previously determined, such as that offered by the 

 Westleton Shingle, before the relation of the other pre-giacial Drift- 

 deposits in the Thames Valley to one anotiier and their relative age 

 could be established. It is for this object that I attach importance 

 to the Westleton Beds last described ; but besides these, which are 

 confined almost entirely to the north side of the Thames, there is 

 another closely allied hill-drift, which I propose to call the " Southern 

 Drift,'' of more limited range and confined chiefly to the south side 

 of the Thames. I at first thought it possible that they might be 

 synchronous*, but now I have come to the conclusion that the 

 Southern Drift is the older of the two, in the sense afterwards to be 

 explained. Both of them are so restricted to outliers, often very 

 small and far apart, and have been so much encroached upon by 

 Glacial Drifts of later date, that the relation they bear to one 

 another is generally much obscured. 



The leading points on which we have mainly to depend are tlie 

 relative levels and the differences in the character and origin of 

 the rock-fragments and pebbles composing the two deposits, and in 



* Much hinges upon their relative levels. These I had originally taken with 

 !\n aneroid barometer, but the new l-inch Ordnance Maps have now furnished 

 me with the more accurate data inquired. 



