XX11 GEOLOGICAL FEATURES 



(ii) Orange-coloured sand with thin layers of blue clay, 

 (iii) Greyish -blue clay ; 

 and the general dip of the strata is towards the south-east. 



Hollows or potholes are very common in the neighbour- 

 hood of Maryborough, and are frequently of large size. 

 They are formed in the chalk by the soaking of water, con- 

 taining carbonic acid gas, through it, which dissolves the 

 chalk: the clay or gravel above fills up the hole in the 

 chalk, and leaves a corresponding hollow on the surface. 

 In some cases it is evident that these hollows have been 

 formed quite recently. One very large and deep one, 

 about 50 yards in diameter, near Ivy's Farm, has been 

 formed since the Wansdyke was built (for the course of the 

 Wansdyke is quite through the middle of it) ; and as the 

 Wansdyke was evidently built for the protection of the 

 people who lived on the south of it (there being a deep 

 ditch everywhere on the north side, and none on the south), 

 nothing could have been easier for the Ancient Britons or 

 Saxons who built it than to take advantage of this hollow, 

 if it already existed, and form their rampart to the south 

 of it. Boulders occur in this hollow, and also in a similar 

 hollow in the midst of the bright-red clay by the side of the 

 Pewsey Road about three miles from Marlborough. 



In the neighbourhood of Marlborough, then, there are 

 many sections of the Upper and Lower White Chalk, some 

 of which contain a plentiful supply of fossils : in the Forest 

 we have some remains of the Plastic Clay series and the 

 Bagshot Sands of the later Tertiary deposits : while capping 

 the tops of the hills, and in some cases covering their eastern 

 slopes, to the north of Marlborough, and extending over a 

 wide area to the south, we have flint-drift or boulder-clay 

 derived from the Plastic Clay series, which at one time 

 covered the district. The clay towards the south-west is 

 of a bright-red colour, with patches of flint and boulders of 

 sandstone, such as are everywhere scattered over the country. 



