The Map That Changed the World (22 page)

The main outcrop of the Jurassic rocks in England.

The Jurassic rocks here had been given names that were almost as sonorous as the villages on which they lay. The Bridport Sands is perhaps rather prosaic, the Yellow Conglomerate a little less so—but what of the Wild Beds, the Red Bed, the Snuffbox Limestone,
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the Forest Marble, the Fuller’s Earth, the White Sponge Limestone, the Scroff and the Zigzag Bed? And if not magical names, then magical appearance: What could be better than the famously extraordinary outcrop, found in a low cliff to the west of the old coastguard station at Langton Herring, which is ten feet thick and made up of nothing less than a solid mass of crushed and flattened specimens of the famous Jurassic oyster,
Ostrea acuminata
?

The village houses here—most of them ancient, many of them thatched, twined with roses, and huddled into cozy valleys—are generally built of limestone blocks that had been hauled from quarries some miles away, and which could not fairly be said to represent the mishmash of rocks below. This offers a sharp contrast to villages in the Cotswolds farther north, where the underlying Middle Jurassic is an inescapable feature of the architecture overhead. My grandparents had lived for a while in Symondsbury, a pretty village lying a mile or so to the west of my main Jurassic track: Their house was built of Portland stone, an elegant building stone (Saint Paul’s Cathedral; the Ashmolean Museum; much of colonial Williamsburg) that actually comes from near the top of the Upper Jurassic, close to its junction with the Cretaceous. I remember seeing in the old house walls traces of the biggest of all ammonites, the famously lumbering
Titanites
, which is a marker fossil for the Upper Jurassic; someone also once pointed out part of the internal cast of the shell of another, odder beast: a small, long, conical, spiral-formed snail
known as
Aptyxiella
, which vulgar quarrymen liked to call a Portland Screw.

To the north of Crewkerne, in Somerset, the Middle Jurassic rocks’ lithologies begin to change. I was now leaving the rush and turbulence of the cyclothemic shore, and coming to a point where the seas—shallow but stable—are more oceanic than coastal, less disturbed, less turbid. The limestones become purer and thicker and more resistant—both more resistant than the rocks in Dorset that are of the same age and, more significantly, more resistant than the rocks in Somerset, Gloucestershire, and Oxfordshire that are younger, and lie on top of them. Because of this the outcrop of the Middle Jurassic everywhere north of the Dorset-Somerset boundary is marked by a sudden, steep hill: As I traveled farther and farther north, so this hill, and the very prominent edge where it rises up and away from the softer rocks below, becomes ever steeper, more and more evident, more and more obvious.

Small wonder that William Smith found the area around Bath the most congenial for his studies. Not only was it an attractive town, jammed with interesting personalities and lively minds: It was also happily sited at a place in the country’s immense geological mosaic in which the Middle Jurassic rocks outcrop in a blindingly obvious way. The general line of their outcrop, which extends all the way north from Dorset to the Humber in Yorkshire, some two hundred miles, is one of the great dividing lines of world geology, once seen, never forgotten. Around Bath, close to where a northbound traveler like me today, Smith two centuries before, first comes across it, it is stupendously memorable.

On the western side of the line are the timid, milquetoast clays and weakling shales of the Lias, of the Lower Jurassic; on the eastern side are the tough, thick oolitic limestones of the Middle Jurassic. On the western side the consequential scenery all is valley and marsh, river course and water meadow, lowing
cattle and in high summer, a sticky, sultry heat. On the eastern side, underpinned by the limestone, everything has changed: there is upland plain and moor, high hills, high wind and flocks of sheep, and in winters fine white snows blowing on what can seem an endless and treeless expanse.

And on the very line itself, at the point where England has tipped itself up gracefully to expose the limestones at its core and to reveal the huge physical contrast between their hardness and the silky softness of the Lias clays below, is a long, high range of hills and cliffs. The line is, for the most part, an escarpment edge that rolls far to the horizon, separating vales and down-lands from high plains and uplands.

We see this line in scores of places. Down at the southern end of the country—the Bath end—we see it where Crickley Hill and Birdlip Hill rise hundreds of feet above the town of Cheltenham. We see it where Wootton-under-Edge (a village set on Lias clay) nestles below the village of Oldbury-on-the-Hill (on Middle Jurassic limestone). We notice it, we
feel
it, when we drop sharply down from it via a dangerously twisting switchback road as we descend westward from the high plains of Snowshill (on the Middle Jurassic) to the antique shops of the clay-valley town of Broadway (on the Lower Jurassic). We can see it unroll over a dozen miles if we drive along the traffic-clogged roadway
of the A46, on the stretch between Bath and Stroud: On going north, everything visible to the left is Lower Jurassic clay, that hunches low to the horizon: Everything to the right is Middle Jurassic limestone and rises high, its edge topped with oaks from which big black crows take in the view of the grassy fields below.

Titanites giganteus
, the largest of the British Jurassic ammonites, can be found up to three feet in diameter.

We see the phenomenon exhibit itself over and over as we rumble northward across the land—we see it through central Gloucestershire and Warwickshire, through Rutland and Leicestershire, across Nottingham and Lincolnshire—so that when, a day or two after I had left the warmth of Dorset, I found myself in the cold of Lincolnshire coasting along the A15 northbound from Lincoln (where there stands a fine Jurassic cathedral, made of just the same-age limestone as that at Wells, down at the far southern end of the outcrop) to Scunthorpe, almost exactly the same held true. To my right rose high limestone plains, buffeted by North Sea winds, dotted with sheep, flat enough and suitably exposed for the building of great Royal Air Force runways, training schools, and hangars. To my left, all was unsubtly different: A yellow cliff fell steeply away, and below it, spread almost flat like an unfolded survey map, lay a long, low valley, thick with farms, populated and cozy. The Middle Jurassic formed the upland landscape to my right; the Lower Jurassic the lowlands to my left.

Seen today, this pikestaff-plain exposition of how the rocks below make the hills above has the standing of a classic of English geology. Not a single guidebook to the geology of England fails to provide an illustration of the great hills at Crickley or Snowshill or the scarp face above the ancient wool town of Wootton-under-Edge. And, knowing now what William Smith had realized, explained, and then colored onto his great map when he saw this vast extent of hillside two centuries ago, “It all seems far too easy!” I find myself tempted to exclaim, as Thomas Huxley cried out on first reading Darwin: “How very stupid not to have thought of that before!” That William Smith
was first to do so was a measure of the man’s extraordinary achievement: to see what others could have seen but never did, to set down on paper what others might have suspected but never felt confident enough to declare.

The rock type that appears most consistently across the hundreds of miles of the Middle Jurassic outcrop is invariably an oolitic limestone. The term comes from the tiny, fish-egg-like concretions of which the limestone is made—and which, in some of the coarser-grained oolites, can easily be seen with the naked eye. Under the microscope the individual ooliths look like pearls, or mother-of-pearl accretions, or those many-layered gobstoppers English schoolboys once knew so well
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—and yet in place of the crystal of aniseed at the heart, there is in the case of the Cotswold ooliths a tiny piece of quartz, or a fragment of shell. Layer upon layer of calcium carbonate had been deposited around this nucleus: Centuries of rubbing and jostling in Bahamas-warm blue seas then kept the slow-forming ooliths small and smoothly spherical; and their color, freshly formed, has invariably either been carbonate white, or else, if there happened to have been more than the usual complement of dissolved iron in the waters of the time, reddish brown.

But hundreds of years of gentle weathering have changed all of England’s Jurassic oolites into the masons’ most magical building stone. The physical nature of the rocks alone has long rendered them a sculptor’s dream. Their bedding planes are straight and well defined, and either the chunks of quarried rock divide naturally into huge freestones, durable and massive and ideal for making big, strong, impressive buildings, or else can be sawed into flat sheets of ashlar, which can be bonded to rougher rocks and give, less expensively, the same harmony and integrity of look and color. Once in a while relics of the old coral reefs are to be found—for Jurassic waters were blue and shallow, much
like the Caribbean waters of today—and make for good building stone too: But they are knobby and ragged, the quarrymen call them hardstone, and most locals think of them as less pretty than the oolites themselves.

This “Cotswold stone” has a well-deserved worldwide reputation: The stones are solid, easy to work, more than amenable to simple carving, perfect for bearing large loads. Above all else, though, the Cotswold stone is quite simply beautiful. The humblest of workmen’s cottages, if fashioned from a Cotswold oolite, is a lovely thing to see—and a huddle of the warm-looking Jurassic stone houses, clustered amicably in some river-carved notch in the meadows, can be so lustrously perfect, so quintessentially English that seeing it brings a catch to the throat.

In the villages that did well enough from the wool trade to evolve into something more than a simple confection of pretty houses, oolite stone was used to make just about everything—the columns, quoins, and mullions of the marketplace, the walls and facings of the cottages, the manor house steps and dove-cotes, the fonts, the choir stalls, the bell towers and the transept floors of the parish church. In one Cotswold town, Bradford-on-Avon, there is a stone bridge with a stone jailhouse at its center point—durable, indestructible, and prisoner-proof.

The walls between fields, too, are built dry-stone, of oolite—and the contrast between a countryside that is apportioned by hedges and its neighbor with its fields divided by walls can be blisteringly sudden: In Lincolnshire, where the escarpment of the Lincoln Edge thrusts up with an immediacy reflecting the narrowness of the outcrop there, hedge becomes wall in just a matter of yards. Glance down from the edge, and every field is surrounded by blackthorn, hornbeam, and dog rose; glance up, and the fields are broken up by long stands of rough and mottled stone, tough, enduring and very, very old.

And yet it is the weathering, more than the simple durability, that provides the magic of the Cotswold oolites. The pale-honey
coloration of newly cut stone, the rich orange and creamy reds of older, well-weathered buildings are quite sublime: The rock has given entire villages—Bourton-on-the-Water, Stow-on-the-Wold, Cirencester, Chipping Norton—an architectural wholeness; and it is a fine piece of irony that the poorer old homes, made of lesser-quality stone, have weathered in far more interesting ways than have the great houses built of the finest, unblemished freestone blocks. The patchiness of the weathered yeoman stone seems to give an extra dash of character, an additional element of charm.

The Middle Jurassic Great Oolite has given a distinctive look to entire cities too, a look that is now a part of the essence, the quiddity, of their very being. How would the crescents and circuses and terraces of Bath look if they had been constructed of some lesser stone? How much less lovely would Oxford be if the oolitic freestones of so many colleges and noble buildings had not been available, and the structures had been fashioned instead from a dour and ancient granite like the stones of Aberdeen, or from a grayish Millstone Grit of the North, of Silurian shales and gray slates from Bethesda, or made from a dull red sandstone like that found in unexceptional Middle English cities like Northampton, Doncaster, and Carlisle? The Middle Jurassic limestones make for a building stone of such gentle loveliness that its Cotswold walls, as J. B. Priestley wrote, know “the trick of keeping the lost sunlight of centuries glimmering upon them.”

There is a little more complexity to the oolite than merely to say that the Middle Jurassic—even the outcrop between Bath and Lincolnshire—is composed entirely of it. There are, as William Smith himself found and described early in his career, at least two types of oolitic limestone represented in the epoch—an Inferior Oolite at the base of Middle Jurassic and a Great Oolite at the top of it. Between the two lies a layer some hundreds of feet thick of a distinctive claylike rock that, since it had the use
ful quality when mixed with water of
fulling
, or leaching the greasy lanolin from lambswool, was (as already noted) called Fuller’s Earth.
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