206 DE. E. L. SHEELOCK AND ME. A. H. NOBLE ON THE [JunO I912, 



Distribution of the Far-travelled Pebbles. 



The sandstones, quartzites, and other pebbles foreign to the 

 ' district have been traced by Mr. H. J. 0. White ^ down the Cher- 

 well, Evenlode, and Windrush into the Thames at Stanlake, and 

 then down the river to beyond the Colne ; and he has shown on a 

 map accompanying his paper that they occur in the angle between 

 the Thames and Colne as far north as a line drawn from Bourne 

 End to E-ickmansworth, and then continue in a band up the Colne 

 towards Hatfield. 



We have found that there is a definite belt of gravel in which the 

 Triassic pebbles are very abundant. Between Hurley and Bourn© 

 End the belt follows the north side of the Thames Yalley with an 

 occasional patch also on the south side ; but at Bourne End, where 

 the Thames bends sharply southwards, the band with abundant 

 Triassic pebbles continues in its north-easterly direction through 

 Beaconsfield, Chalfont, and Bickmansworth to Watford, where it 

 passes beyond the area that we have mapped. The belt of gravel is 

 about 3 miles wide, but widens to 4 miles between Hedsor and 

 Hedgerley. South of this belt there is a marked falling-off in the 

 number of Triassic pebbles, although a few can be found everywhere. 



The fact, that the Triassic pebbles are plentiful in the gravels 

 which border the Thames as far down as Bourne End, is strong 

 evidence that the river introduced them into the belt of gravels 

 between Bourne End and Watford. The Clay-with- Flints and 

 associated Gravels do not contain Triassic pebbles (except ocoa- 

 sionally near the junction with the Fluviogiacial Gravels), so that 

 they were not derived from these deposits, and the only sources 

 from which the pebbles can have come are down the Thames or 

 down the Colne from above Watford^ The fact that the level of 

 the gravels falls from the Thames towards Watford, and that the 

 number of the pebbles also falls off in this direction, leaves us in no- 

 doubt between these hypotheses — they came down the Thames. 



lY. FOEMEE COUESE OF THE ThAMES. 



We consider, therefore, that the Thames flowed at one time along 

 the foot of the Eocene escarpment from Bourne End to Bickmans- 

 worth, and that the Colne between Eickmansworth and Watford is 

 a part of the old channel. From this channel the river was diverted 

 southwards by an ice-sheet coming from the north and north- 

 west. 



At the present time the boundary of the Eocene is buried under 

 the Fluviogiacial Gravels between the Thames and the Colne. 

 There is, however, a well-marked escarpment west of Bourne 

 End, while south of that place is a gorge where the Thames turns 

 southwards. Between the Thames and the Colne, we think that 



1 ' On the Origin of the High-level Gravel with Triassic Debris adjoining 

 the Valley of the Upper Thames' Proc. Geol. Assoc, vol.'^xv (1897-99) 

 pp. .157-74. 



