54 LONDON DISTRICT. 



terminology is the level of the shelf or terrace in the * solid ' 

 rock on which the gravels rest as a floor ; this figure is not always 

 readily obtainable and, in a single spread, increases with the dis- 

 tance from the river ; when the terraces are traced further up 

 the Thames or up the tributary valleys confusion is inevitable. 



When the systematic re-survey of the London District on 

 the 6-inch scale was undertaken in 1910, beginning from the 

 neighbourhood of Maidenhead, it was decided to adopt a new 

 nomenclature which should involve no presuppositions. Gravel 

 terraces occur in that district at Boyn Hill some 70 ft. and at 

 Taplow some 20 ft. above the local river level. At both localities 

 flint implements occur, and at Taplow Station 1 pit a fine series of 

 mammalian fossils was found. 2 



These were chosen as type localities and the names Boyn 

 Hill or Boyn Terrace and Taplow Terrace were adopted, and 

 will be found on the 6-inch maps and in the New Series One-inch 

 Sheets 255, 256, 269, 270; the lowest gravels lying in the valley 

 bottom were called Flood-plain gravels. One result of the re- 

 survey has been to indicate the existence of three, and only 

 three, main divisions of the river deposits, and in consequence 

 we can safely speak of the Upper (Boyn, or 100-ft.), Middle 

 (Taplow or 50-ft.) and the Low (Flood-plain) Terraces. 



The Upper Terrace. On the north side of the Thames the 

 Upper Terrace is found south of Iver and Hillingdon in the west. 

 Its lower limit is here about at the 100-ft. contour, and is also 

 the upper limit of the Middle Terrace, the division being marked 

 by a sudden drop in the level of the London Clay beneath; a 

 similar feature occurs between the Middle and Lower Terraces 

 further south and here and there causes an outcrop of the London 

 Clay, but the line with which we are now concerned is frequently 

 obscured by the subsequent descent of the upper gravel and by 

 the overlap of the later Brickearth ; excavations have revealed 

 the ' buried' cliff near Osterly Park. 3 The upper limit is clearly 

 seen at Iver and at Hillingdon Heath, where it runs up to about 

 160 ft., but the deposit there may belong more to the Come than 

 the Thames ; in the former valley and that of its tributary, the 

 Gade, the Upper Terrace rises as high as 280 ft., but becomes 

 merged in the Glacial Gravels. The Brent, on the other hand, is 

 a newer stream and has cut through the Upper Terrace of the 

 Thames, which is also interrupted by the low ground round 

 Shepherd's Bush. The gravel of Castlebar Hill, Baling, is pro- 

 bably an outlier of this terrace, but that on Hanger Hill (shown 

 on Sheet 1 of the map surrounded by Brickearth), appears to be 

 Glacial gravel capping an outlier of Clay gate Beds. Further 

 east this terrace has been wholly removed by denudation except 

 at the outliers of Islington and Highbury. The gravels of the 



1 Formerly called Maidenhead : not the present Maidenhead Station on the 

 Berkshire side of the river. 



2 Owen, B., Quart. Journ. Geol. Soc., vol. xii, 1856, pp. 124-130; 

 Dawkins, W. B., ibid., vol. xxv, 1869, p. 198. 



s Brown, J. Allen, Proc. Geol. Assoc., vol. x, 1888, p. 362. 



