OF EAST EEEKS AND WEST SURREY. 563 



(3) The more ancient arterial line of drainage of southern 

 Eocene England * having been obliterated by the silting up of the 

 Eocene estuary f by the Bagshot Eeds, and subsequent regional 

 movements of the Mesozoic strata to the south, the modern Thames 

 Basin began to be outlined, although probably its main line of 

 drainage did not coincide exactly with the modern valley of the 

 Thames. 



(4) Distinct evidence of gl aciation at much lower levels than the 

 plateau- gravels (210 to 240 ft., o.d.) being now to hand in the district, 

 and this being presumably the work of floating ice in the old Thames 

 Straits, we may (in the absence of distinct traces of glaciation at 

 higher levels) assign something like 200 feet (vertical J) of the 

 excavation of the present valley- system of this part of England to 

 a period intervening between the deposition of the plateau-gravels 

 of the Southern Drift and the Glacial Epoch at its maximum. 



(5) The deposition of these Plateau-gravels appears to have 

 occupied a considerable portion (perhaps the whole) of the Pliocene 

 Period ; the only data available for fixing the time of the initiatory 

 stage of their deposition being the inconclusive evidence furnished 

 by the well-known Lenham deposits. The results of my own work 

 and of the independent researches of Prof. Prestwich are thus seen 

 to confirm the suggestions of Sir A. C. Eamsay §, made years ago, 

 as to the pre-glacial age of these Plateau-gravels, and of the present 

 valley-system of Southern England. 



Discussioisr. 



Mr. MoNCETON had no doubt that the block of quartz referred to 

 in the paper came from the Plateau-gravel. Everybody was agreed 

 as to the materials of the Plateau-gravels having come from the 

 south-east. Such contortions as described were generally accepted 

 as evidence of snow or ice. The age of the Plateau-gravels was 

 admittedly doubtful ; and, if possible, the Author had rendered 

 their age more unsettled than ever. He (the speaker) inquired 

 as to whether the Author used the term Boulder-clay with chrono- 

 logical significance, and what evidence was brought forward as proof 

 of the marine or fluviatile origin of the Plateau-gravels. 



Prof. Etjpert Jones said that Mr. Godwin-Austen referred the 

 Plateau-gravel to an early Tertiary age. He had lately seen, at 

 Cuckmere, conditions of the shingle analogous to those which 

 Dr. Irving had suggested for the Plateau- gravel. He believed 

 that this, as well as the lower gravel, exhibited the effects of ice- 

 action. 



* See " The Stratigraphy of the Bagshot Beds, &c." Quart. Journ. Geol. Soc. 

 Yol. xhv. pp. 181, 182. 



t This question has been discussed in my papers on the Bagshot Beds in the 

 Quart. Journ. Geol. Soc. vols, xliii. & xHv. 



I To this it would be easy to attach an exaggerated importance, if we over- 

 looked the easily-destructible nature of the Bagshot strata. 



§ ' Physical Geography and Geology of Great Britain ' (5th ed.), p. 533. 



