THE BAG SHOT SANDS. 49 



side, giving a much lower level than that of the summit-road, 

 and leaving some of the older trees standing on isolated mounds 

 with the roots in many cases exposed. The picturesque clumps 

 of Scotch firs which have attained large dimensions, attest the 

 suitability of these trees to the ground on which they grow so 

 well, while the oak and elm are to be looked for on the lower 

 levels where clay and sandy clay form the subsoil. 



As has been previously stated, the capping of sands on 

 Hampstead Heath is an outlying patch of the Bagshot S^nds, 

 quite cut off from the main mass of the formation which occu- 

 pies an extensive area in Surrey, Berks, and Hants. The 

 Bagshot Sands are everywhere very sterile, and are chiefly 

 covered with heaths and commons and pine-woods. They 

 give to us those extensive wastes round about the junction 

 of the three counties just named that have been found so 

 useful for military and other purposes requiring large areas of 

 ground of little or no agricultural value. 



The " Hampstead Outlier," as it is called, is one of the 

 best examples of an outlier since it is so distinct from the 

 surrounding beds, so limited in area and so well exposed. Its 

 presence, besides marking the top of the maximum thickness 

 in this area of the underlying London Clay, indicates the 

 original extension of the great sheet of the Bagshot Sands 

 to the north side of what is now the Thames Valley, and the 

 amount of denudation that has taken place between this point 

 and the Surrey beds, by which the present levels and form of 

 the ground of the London area have been produced. 



From this it is seen that in the middle of the Thames Valley 

 about 400 feet thickness of solid material has been removed 

 to form the present bed of the river. The amount of material 

 removed lessens as the northern or southern boundaries of the 

 Thames Valley are approached, and at Hampstead, Highgate, 



4 



