LOWER CHALK—CONDITIONS OF DEPOSITION. 
345 
in progress elsewhere. But in North Wiltshire, from near Caine 
to Swindon, there is an abrupt transition with distinct signs of 
erosion. 
Northward, again, through Berkshire, Oxfordshire, Bucking¬ 
hamshire, and into the western part of Bedfordshire there is 
always a passage from the Selbornian to the Chalk Marl, so that a 
large area here was out of the track of the strong eroding currents. 
Beyond this, however, is another tract at least fifty miles in 
length, along which the Gault presents a surface of erosion and is 
overlain by the Cambridge Greensand or Nodule-bed (see p. 194). 
This eroded surface probably terminates somewhere below the Bens 
on the borders of Suffolk and Norfolk. North of it we have an 
area of slow deposition,with a complete and uneroded representative 
of the Gault overlain by a similarly condensed equivalent of the 
Chalk Marl. In Lincolnshire and Yorkshire also the deposition of 
calcareous sediment was continuous, so that there is no break, 
but a complete passage from “ Bed Chalk ” to the Chalk proper. 
From the above resume it will be seen that the most marked 
signs of current action, with removal and sifting of material, occur 
(1) in Kent and Sussex, (2) in Dorset and Devon, (3) in North Wilts, 
(4) along the tract occupied by the Cambridge Greensand. It 
will be worth while seeing if the last can be traced to any distance 
beneath the chalk, and, if so, in what direction it runs. 
A boring made at Culford in 1890* is important as showing 
that the eroded surface of the Gault does not extend far to the 
east of Cambridge and Soham, for in this boring the change from 
Chalk Marl to Upper Gault seems to have been like that near 
Stoke Ferry, in Norfolk. 
The next nearest place where a boring has been made through the 
chalk is at Ware, about thirty miles S.S.E. of Cambridge, and 
twenty miles south-east of the termination of the outcrop of the 
Cambridge nodule bed in Bedfordshire. It is interesting to find 
that a sample, believed to have come from the base of the chalk 
below Ware, has all the characters of the nodule bed as formerly 
to be seen near Sharpenhoe in Bedfordshire, f 
Nothing comparable to this nodule bed has been recorded from the 
boring at Cheshunt, but the samples preserved did not include one 
from the actual base of the chalk, so that one cannot positively state 
it to be absent there, more especially as it seems to be present under 
London. Describing the Chalk Marl in the boring at Meux’s Brewery 
(Oxford Street), Mr. Whitaker observes J ‘fin a fine core from the 
base, at the office [of the Geological Survey], I saw a good specimen 
of the Cambridge phosphate bed (said to be six inches thick) 
with greensand and nodules passing up into regular Chalk Marl.” 
* See Quart. Journ. Geol. Soc., Vol. 1. p. 489. 
t Loc. cit., p. 504. 
+ Geology of London, VoL ii. p, 167,, Mem. Geol. Survey, 1889, 
