CHAPTER XIII.

_THE GEOLOGY OF THE HEATH._

The appearance of the Upper Heath, as that portion of it beyond Jack Straw’s Castle to the north-west is called, shows that the purchase of it for the sake of its preservation was not a day too soon, while as far as preserving the primitive beauty of the Heath, it was years too late.

The surface, originally flush with the paddock near the North End Hill, has been delved by sand and gravel diggers into a series of pits and hollows, with corresponding mounds and hillocks. At one period (1811), owing to the multiplicity of building operations going on, upwards of twenty loads a day passed through Hampstead, besides the quantity taken away by other roads.

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.

In the lowest part of the deposit it becomes more clayey, and passes gradually to sandy clay, and eventually to the stiff blue clay called London Clay. Many well-preserved fossils are found in the sandy clay, which proves that the deposit was formed 50 fathoms below the sea-level; while the fossils of the London Clay indicate a much deeper sea.[224]

The lowest portion of the sandy clay is known by the appearance of swampy ground, and by the oozing out of the springs, as in Well Walk, in Conduit Fields, and at North End. It is the property of clay to hold up water, and the lower part of the sand, through which it percolates, lying horizontally on the clay, and becoming very full, the water comes out at the edges of the hill, especially at the places indicated. The sandy clay leading down to the London Clay is about 50 feet thick, and from that at Child’s Hill beautiful marine shells, quite perfect, showing that they had neither been rolled nor drifted, were found at a depth of 30 feet in an excavation for drainage in the Finchley Road (Child’s Hill, 1872). Below this comes the stiff London Clay, about 350 feet thick.

The chalk at Hampstead Hill is another proof of its submarine formation. This is many feet thick, and is pure carbonate of lime, composed of minute sea-shells, and must have taken an immense period of time to form. There have been found in it hard portions of animals similar to those which now dwell in the sea. So many evidences exist around the British Islands of change of levels, both by elevation and depression, that there is no improbability in supposing that Hampstead Hill has through past ages been gradually raised from below the level of the sea, and at times has been again depressed, which change geologists believe to have taken place more than once, the hill not taking its present form till after several upheavals.

The changes of temperature must have been as vast as the geological ones. Tropical animals—large elephants, rhinoceroses, hippopotami, etc.—are said by Professor Owen to have inhabited the neighbourhood of Hampstead; and though no evidence remains here of the glacial period, icebergs floated at Finchley, and left their deposits in the shallows of the sea that covered it, and doubtless at that period Hampstead was covered with thick ice. The fossil nautilus, sharks’ teeth, and the plates and spines of echini,[225] have frequently been found, the latter in gravel-pits upon the Heath. Modern geologists have stated that the sand at the top of the Heath is only a small patch, very irregular in shape, and that there was another patch on the top of Highgate; and it is suggested that perhaps these were formerly connected, but that the depression of the ground at Caen Wood may have swept the sand from them. Park, on the other hand, observes ‘that vast quantities of sand exist at Hampstead, the Heath being covered with it at an average to the depth of 10 feet, though in some places it is more than 25 feet in depth, notwithstanding the length of time it has been supplying the Metropolis and intermediate villages.’ Could both be thinking of the same stratum?

That the Heath is covered with sandy gravel (in fact, the Heath is confined to the sand) is sufficiently apparent to the ungeological eye, especially in this rugged and denuded portion of it. It has been a vexed question with the artists and the conservators of the Heath whether to fill up these irregularities of the surface or leave them to Nature’s healing. Already, taking advantage of the past year or two’s rest from aggression, she has covered the scarred places with her green mantle, and crowding fronds of common brake have taken to grow on the graves of its old habitat. Great spaces amongst the gravel-pits have been brilliant with the glittering flowers of the common broom, and where the unquenched springs still drain themselves into pools and shallows, stocks of willow-wood have in some instances been driven, which have taken root and put forth branches, and in a few more seasons will be vigorous trees.

Our hope is that the present conservators of the Heath, to whom great praise is due for the visible improvement in its appearance, will be patient with this seemingly most hopeless portion of it, and leave the rest to the great Mother’s care. In time the rugged superficies will round and soften, and the hollows be converted into bosky dells, tangles of woodbine, wild-rose, and arching brambles. We have already seen indications of the return of _Erica cinerea_ and _E. tetralix_, once common on the Heath, and the tufted stems and silvery lilac flowers of the indigenous heather.

If loving hands a little after harvest-time would bring an alms of hips and haws and mountain-ash berries and drop them carelessly about the turf, the birds would scatter them, and help to bring back beauty to the Heath, that wild beauty that is Nature’s own, and, though quite unpremeditated, is ever in agreement with its surroundings.

For the geological part of this chapter I am indebted to notes taken of a lecture on ‘Hampstead Hill in Past Ages,’ delivered by C. Evans, Esq., F.G.S., in Rosslyn Hill Schoolroom, March, 1872.