Hard grounds
The ground in this part of the wood is crunchy under my boots. Beneath a few of last year’s fallen leaves and under the questing loops of bramble shoots there appears to be nothing but rock. I am attempting to dig a hole to explore the surface geology, but my spade refuses to make any progress. Its blade twists and complains against a barrier of stones. I will have to employ my geological hammer to solve the problem.
The pick side of the hammer starts levering up lumpy flints, some bigger than my fist. They leave the damp ground reluctantly, with a sucking noise. Where I hammer downwards into the growing hole, sparks fly where steel meets flint. Briefly, there is a smell of cordite; in the days of flintlock pistols that smell would have been a familiar one. Flints were used to strike the spark that ignited gunpowder before a shot could be made. Our flints are embedded in reddish ochre clay that tries to hold on to them, clay that can easily be rolled into a coherent ball between the palms of my hands, and sticks to the fingers. The exterior of most of the flints is white when wiped clear of its clay coat, but where the hammer has shattered one of the larger flints its interior is strikingly black, and mottled in patches. It is a hard rock, but a brittle one shot through with flaws. Much of the wood is effectively floored with flint. Of the chalk of the Chilterns there is no sign.
Just down the hill beyond the Fair Mile I know that chalk underlies everything. When the dual-carriageway road was repaired great masses of the white rock were dumped on the side, and I picked out a typical, conical fossil sponge called Ventriculites from the rock pile. Even within Lambridge Wood, further downslope towards Henley, a mysterious excavation known as the Fairies’ Hole (marked on even the oldest maps) is undoubtedly dug within the white limestone. The rock that makes the whole range of hills, ‘the rock that bore them up all on its back’, as H.J. Massingham said, is an understory of chalk. Within the chalk, hard flints form discrete layers, but they never dominate completely. This flint was ultimately derived from fossil sponges within the chalk that had internal skeletons made of silica struts. The silica was first dissolved, and then re-deposited in flinty layers as the original chalk ooze gradually hardened and transformed into the rock we see today. Whatever underlies Grim’s Dyke Wood on the higher ground evidently also lies on top of the chalk formation, but is largely made of flints derived from it, all stuck in a matrix of sticky clay. This deposit is called, unsurprisingly, clay-with-flints, and in the wood the flints are dominant.
Clay-with-flints caps the chalk in many parts of the Chiltern Hills.10 It is the product of many millennia of slow solution and weathering-away of the chalk; it is what is left behind when everything else is removed. Chalk is weakly soluble in rainwater, which is why water derived from an aquifer in the Chilterns leaves a limescale deposit behind in a kettle. After a very long time, as the chalk naturally disappears the originally scattered flints become concentrated. Flint is insoluble; in fact, this form of silica is well-nigh indestructible. It can be tossed into rivers or buried in gardens for centuries, and emerges unscathed. It will outlast the Chilterns.
To estimate the thickness of the clay-with-flint capping I walk slowly up from the end of the Fair Mile to Lambridge Wood along Pickpurse Lane (see comments on highwaymen), digging with my hammer into the bank until the telltale milkiness goes out of the soil. There are other signs to look for. Old man’s beard (Clematis vitalba), the nearest thing in the British flora to a liana, only grows on chalk – it will not tolerate clay-with-flints. Wild marjoram is no more forgiving. Plant roots sense chemistry with the exquisite palate of a connoisseur. Both the indicator plants grow in abundance near the bottom of the lane and fade away upward. By the time all evidence of chalk has disappeared I conclude that very roughly twenty feet of clay-with-flints must lie above. That is sufficient to make the thin soil on the high ground neutral or acidic compared with the alkaline soils on the slope and in the valley bottom. This saddens me, for many of the more glamorous plants love chalk: the whitebeam tree with big simple leaves with shining undersides; cheerful yellow St John’s wort; and many an orchid. I shall just have to live without them – I cannot argue with geology. Now I also know why our footpaths can become like quagmires after too much rain. That layer of impermeable clay does not drain well; it likes to make ponds. Some corner of our wood will always be damp.
Back to Grim’s Dyke Wood. I decide not to try to excavate much more of the recalcitrant stony ground. Instead I shall use the holes I have made to put down beetle traps, burying a few cups half-filled with lethal Dettol to ensnare night crawlers. As I tidy up, a different stone surprises me. Lying on top of the ground by a beech trunk is a pebble the size and shape of a goose egg. It is purple, and it is certainly no flint. Under my hand lens I recognise it immediately as hard sandstone. I soon see more examples of a similar cast, liver-coloured, always rounded off to make satisfactory hand specimens, by which I mean something that sits easily in the palm. They are all strangers. There is no rock formation I can think of in the Chiltern Hills, or in the Vale of Aylesbury beyond, or even further afield beyond Oxford, that might produce such pebbles. They have all their corners chipped off until they are satisfyingly elliptical in outline, and smoothly rounded at the corners. This is a form sculpted by long sojourn in a lively river; erosion has knocked them into shape little by little, polishing repeatedly over a very long time. How could they have got here, into the middle of our beech wood? There are other strangers too. A white pebble that might be a pigeon’s egg, judging from its shape and size; it’s another form of silica – resembling flint, but with a dense, swirling milky whiteness. Vein quartz, I will wager. It might have originated from a vein within granite or snaking along a fault fracturing other rocks. There is no source for such vein quartz anywhere around here. Strewn on top of the clay-with-flints are a bunch of lithological vagabonds from afar.
I decide to investigate further. At the Natural History Museum a skilful colleague cuts sections through my errant pebbles. Microscopic examination should show what they are made of, and reveal the secrets of their derivation. The samples are sliced using a diamond saw; then a thin sliver is mounted on a glass slide and reduced in thickness so much that light can penetrate the minerals that make up the rock; they can now be examined under a petrological microscope. I learned my microscopy skills as an undergraduate in a dusty laboratory in Cambridge, and distant memories stir as I stare down the eyepiece.
The vein quartz pebble proves to be typical. Under the microscope it shows as an irregular patchwork of grey or slightly yellowish crystals, with trails of tiny bubbles. It could have originated from several geological sites. However, one sample has several good pieces of similar-looking rounded vein quartz embedded within a chunk of the sandstone, like plums in a pudding. Maybe the quartz pebbles were derived from the same sandstone formation, only a part of it that was much coarser – a conglomerate, in geological terms. The pebbles must have been incorporated into the sandstone from some still older source. The sandstone itself is curious and distinctive. The individual sand grains are clear enough as masses of rounded outlines under the microscope, and they are of similar size to those that might be found on a beach today. But they are glued together by dark-red cement, without doubt full of iron. This is the mineral that gives the pebbles their rich red colour. The sandstone is recognisable, and it can be run down to its source. The pebbles must be Triassic in age (about 235 million years old), and they come from the English Midlands.11 The old name for them was from the German – Bunter sandstone12 – and they date back to a time when Britain was hot and arid and the geography of Europe had an utterly different cast. As for the indestructible milky quartz pebbles, some of them originated from the erosion of still older rocks long before they in their turn became incorporated into the Bunter sandstone; they might be as old as a billion years. Enmeshed under our own beech roots we have pebbles that account for a quarter of the history of the earth; and they arrived in the Chilterns by water, without question.
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