LOWER CHALK —NORTH WILTSHIRE. 
155 
CHAPTER XII. 
THE LOWER CHALK IN NORTH WILTSHIRE. 
General Description. 
The Lower Chalk of the Vale of Pewsey is a continuation of that 
between Westbury and Laving ton. Its outcrop passes from Laving- 
ton north-eastwards to Hrchfont, and then eastwards along the 
southern side of the Vale of Pewsey, entering the valley of the Avon 
as far as Chisenbury and the Collingbourn valley as far as Colling- 
bourn Kingston. From Tidcombe, at the head of the Vale, the 
outcrop curves to the north-west, and becomes much narrower in 
consequence of the steeper inclination of the beds, which continues 
as far as Hewisli. West of that the arch of the anticline is much 
flattened, and the Lower Chalk spreads out more over the Selbor- 
nian by the Altons and the Cannings. There are, moreover, outliers 
still remaining on the Selbornian area at Woodborough, Etcliil- 
hampton, and near Potterne. 
The main outcrop passes below Roundway Hill, near Devizes, 
and thence northward above Heddington and Calstone to Cherhill, 
east of Caine. Beyond this the Lower Chalk forms a separate 
escarpment, and its higher beds occupy a considerable portion of 
the plateau which intervenes between this escarpment and that 
formed by the Chalk Rock and higher part of the Middle Chalk. 
These features continue as far as Chiseldon, but beyond that the 
two escarpments come nearer together and the width of the ground 
occupied by the Lower Chalk becomes narrower. 
The thickness of the Lower Chalk is probably about 240 feet 
throughout the whole area, this figure being arrived at from the 
levels on the six-inch maps at several places where the top and 
bottom of the formation are within a short distance of one another 
and the dip is slight. 
There are no complete continuous sections through the Lower 
Chalk in this area, and, so far as we know, it has not been very dili¬ 
gently searched for fossils, so that the upper limit of the Ammo¬ 
nites varians zone has not been determined. 
There is very little marly chalk in the zone of Am. varians, most 
of this zone consisting of blocky chalk, with occasional harder beds 
of grey chalk. It includes beds of whitish siliceous chalk, much of 
the siliceous matter consisting of sponge spicules and minute globules 
of colloid silica, the proportion of such silica sometimes amounting 
to over 20 per cent, of the mass. This siliceous chalk often contains 
lumps of bluish-grey siliceous stone, which may be regarded as 
flints which have not been completely silicified. 
