Chweldoh to CoUingboume. 



101 



and so when these beds are found the lower chalk might yield a 

 good deal of water. 



But neither this division nor that of the middle chalk can be 

 compared with the upper chalk for purposes of water supply. The 

 many layers of flint and the fissures so plentiful in this formation 

 assist percolation, and the water sinks through the chalk till the 

 level of saturation is reached. This level rises and falls with the 

 amount of rainfall. 



Agriculturally- the upper chalk is very important, forming a good 

 arable soil, especially when covered by the red clayey flinty soil 

 known as the clay-with-flints, and only found on the upper chalk 

 in this district. The carbonated rain water, the carbonic acid 

 being derived from decaying vegetable matter, percolating through 

 the flinty chalk, removes the carbonate of lime and leaves the 

 clayey part of the chalk as a red clay stained by iron. 



Thus, from an agricultural point of view, the lower chalk forms 

 the heavy arable land, the middle chalk the down land, and the 

 upper chalk, when bare, the light arable land. 



The clay-with-flints and the Tertiary debris often fill pipes 

 many feet deep. Mr. Codrington, in his paper on the Greology 

 of the Berks and Hants Extension and Marlborough Railways, 

 describes some pipes 30ft. in depth in the cutting between Savernake 

 and Marlborough, near Wernham Farm. 



Mixed with the clay-with-flints is to be found a good deal of 

 mottled clay, the remains of the Tertiary beds. 



In the brickyard on the top of Salisbury Hill, near the Marl- 

 borough Water Works reservoir, are some very good sections of 

 both these clays. The pipes are lined with a thin coating of clay- 

 with flints and filled with a considerable thickness of Tertiary 

 clays, the debris of the Tertiary beds. 



It is this clay-with-flints and the existing outliers of the Tertiary 

 beds, forming a soil so favourable to the growth of trees, to which 

 Savernake Forest owes its existence. The forest area roughly 

 marks out the clay area, and though to the west of the forest there 

 is a considerable clay area bare of trees, this is because they have 

 been cut down. The forest must once have extended as far 



