Looking at the ravaged Heath as it appeared in 1872, it would seem as if this wholesale devastation had been going on ever since, without reference to anything but the market value of the deep layer of gravelly sand which geologists tell us overlays the Heath in places to the depth of 80 feet. No doubt the barren appearance of the surface east of the Spaniard Road and in the vicinity of the Vale of Health may be attributed to the removal of this gravelly substratum till the clay was reached, which formed the vari-coloured hillocks that used to make quite a feature of this portion of the landscape. Subsequently, as we have seen, the highest part of the Heath was treated as one huge gravel-pit, the purchasers of which dug out their loads any and every where, encroaching within my memory on the Fir-tree Avenue, in front of the historic houses at Park Gate, as this entrance to the Heath continues to be called; and, not content with delving it in the open, the purchasers were permitted to ruthlessly dig out the sand from under and between the roots of the fine old trees, undermining many of them, and leaving them a prey to the first tempest. In this way nearly all the trees on this part of the Heath have suffered; and to this cause may be attributed the fragmentary condition of the Stone Pine Avenue, and the curious exposition at one time of the efforts of some of the remaining ones to support themselves by sending pile-like roots into the ground on the side on which they are most exposed to tempests. Fortunately for their existence, the Board of Works have taken steps to preserve their weird beauty to the Heath, and protect the groups of elm and ash and other trees, which so long as the season of leafage and blossom remains to them will literally keep green the memory of that lover of Nature, the planter of the majority of them, Mr. Turner, of Thames Street. Naturalists and geologists may still find here abundant materials for their studies,[222] and the geology of Hampstead Heath would in capable hands prove a most interesting chapter in its history. But the writer is not a geologist, so must be content to summarise what others have said, or written, of it. Time was when a sea a hundred fathoms deep rolled over the present site of London and the lands around it.[223] Evidence of its having been above Hampstead Hill is found in the deposits it left on the summit of it. On the highest part of the Heath there lies a horizontal bed of light-coloured ferruginous sand, mostly coarse and gritty; but an admixture of fine sand and thin bands of loam occurs in places, which, like the sand, is destitute of fossils.