The rocks here in Kent could be shown by their fossils to belong not merely to a younger geological stage or a younger epoch than those back in Dorset. They would belong to an entire geological period, the Cretaceous, that was a full fifty million years younger than these Jurassic rocks that stood beside me. To cross the southern coast of England, west to east, is thus to travel forward—and at breathtaking chronological speed—in a self-propelled time machine. With every few hundred yards of eastward progress one passes through hundreds of thousands of years of geologic time: A million years of history go by with every couple of miles of march.
Here in this particular corner of Dorset, at the coastline by the tiny port of West Bay—which is where we as schoolboys frolicked in the sea—one huge cliff made of sandstone and topped by a cap of limestone dominates all the scenery. It stood to the east of the tiny harbor and was known, somewhat unimaginatively, as East Cliff. And it was exactly here, cold and swim weary back in the 1950s, that I found that first ammonite. Some forty years on and I had walked down to the sea once more,*and I had turned to face eastward—and the selfsame cliff rose dramatically ahead of me. It looked quite unchanged—solemn, brooding, and immense.
I walked over to its base, past the bundled-up vacationers and the few pet walkers braving the late spring chill. In part I hoped I might find another fossil; mainly, though, I just wanted to gaze up at what must have enthralled William Smith as he contemplated the beginning of his fifteen-year mapmaking tour, as he reached the starting point of his thousands of miles of trekking along and across the outcrop of the English Jurassic.
East Cliff rose up in a great domed cathedral curve, rising from where two small streams had eroded it down to sea level on its western and eastern ends. At its center it was maybe 130 feet high, vertiginous, and quite sheer. Its face was composed of thin bands of a hard calcareous sandstone that alternated with thicker bands of an orange-brown rock that was much softer and more easily weathered. From afar this gave the whole cliff a quite dramatically striped appearance, with the stripes all sloping gently to the east along the dip of the outcrop. The layers of sandstone progressively thinned higher up on the cliff face—until, at the very top, there suddenly seemed to be a layer of a rather different rock, a thick band of a mustard-colored limestone that over-hung the rest of the cliff and had evidently proved alarmingly dangerous to whoever farmed or walked or played golf on the meadowland at the top. Chunks of this rock lay littered below on the beach—showing where either human beings had deliberately knocked overhanging blocks of it down from the summit, or where erosion had caused it to tumble.
It was from within the shattered remains of a huge boulder of this particular rock that I had found my polished-looking ammonite all those years ago. Now, nearly half a century on, I looked for another. I looked long and hard, but after an hour or so admitted defeat. I couldn’t find one anywhere—although I did find one or two fragments of very different-looking specimens, bits of ammonites that did not look polished at all, and which, to judge from the sandy limestone particles adhering to them, seemed to have fallen from some of the bands of rock lower down the cliff face.
In finding these I was reminded of the long debates we used to have in our Oxford paleontology classes, about whether smooth-skinned ammonites, with their decidedly streamlined appearance, could swim any faster than those that had more ornamentation and roughness about them. The general belief was that it made no difference, and that an ammonite—no speed demon at the best of times—lived its life bobbing near the surface of the sea and using the arms that hung from its backward-facing aperture merely to rock itself gently back and forth, to grab hold of passing morsels of food and stuff them into its jaws, where they were roughly dealt with, chewed up, and passed on into the digestive tract.
Nonetheless, the guidebooks I had with me on my second visit readily identified that first ammonite. The smooth-skinned cephalopod turned out to have been a fine example—how I wished now that I had managed to hold on to mine!—of Leioceras opalinum, a creature that defines by its very presence a junction point of immense importance within the English Jurassic.
The rocks that customarily lie immediately below what is called, eponymously, the opalinum zone, belong to the upper part of the epoch known as Lias, and which itself is chronologically a part of the lower part of the geological period known as the Jurassic. Then again, those rocks that occur (at least, occur in this part of the world) immediately above where this fossil is to be found, belong to a formation called the Inferior Oolite, and which lies within the epoch generally known as the Middle Jurassic.* L. opalinum, the small, smooth, reddish disk of the ammonite I had once held thus forms the boundary marker between the Lower and the Middle Jurassic, between the Lias and the Dogger.
This smooth-skinned ammonite, the palm-handy stone I had once contemplated skimming across the surface of the Channel, records one long instant of faraway time. It was an instant that can be proved, by the radiochemical techniques that have so advanced the business of chronostratigraphy, to have passed more or less exactly 178 million years ago.
The fossil that I had once held in my almost-six-year-old hands was one of the most powerful keys that would unlock the secrets of Jurassic time. Lioceras opalinum, indeed, was a fossil—one of very, very many in his collection—that William Smith, with his theory of fossil uniqueness, would come to know very well indeed. It was a great shame that I had lost it: I would have liked to have carried a specimen with me, as talisman, and try as I might, I could not find a replacement.
Leioceras opalinum, a defining smooth-skinned ammonite of the Lower Jurassic.
To have discovered one would have been ideal; and yet my memory of that first one remains comfortably powerful enough. As I walked back up the cliff to the parked car it occurred to me that my hero and I were now subtly connected by this single small lozenge of limestone and calcite, by the smooth and silkily beautiful physical object that he had once held in his hands, much as I had done all those years later. To Smith back then, and to countless other inquiring collectors since, the little fossil was very much more than simply a four-ounce reminder that tropical oceans had existed in the England of 178 million years ago. It was a symbol also of the beguiling magic and mystery of the science of modern geology, and provided a cozy link between the past and the present, between the extremes of the ultramodern and the ultra-ancient.
The Carboniferous period in Britain, which began 360 million years ago, was generally a very lively time. It was an age characterized by torrid wet heat, by shallow seas and endless swamp, by thick rain forests crawling with slimy amphibians, a time of huge dragonflies and tall ferns, of warm deltas with pools that supported trilobites and shellfish. And then, with what can only be described as a fit of geological suddenness, everything changed, and all the life that was richly abundant dwindled to nothing, mysteriously withered away. In the Permian and Triassic periods, what is now the continent of Europe was dominated by endless sandy wastes, blasted by hot dry winds. Lifelessness, aridity and blistering heat suddenly took the place of all that Carboniferous moisture and fecundity.
There is an explanation for all this, an answer satisfactory to all who have wondered, and that has only been newly found, buried in the arcane complications of plate tectonics.
The early world was a terrifyingly volatile place. It was always mobile, its crustal blocks caught up in violent swirls of ferocious movement. It now seems clear that during the Permian one of these immense million-year-long swirls saw to it that all the protocontinents were briefly fused together—yet again; they had been fused many times before. In doing so this time they formed the now-familiar supermass known as Pangea, the continent that was the true beginning of earthly everything.
There were very few internal seas within Pangea: Such oceans as existed in Permian times lay principally at the peripheries of the giant landmass. Within, arid and windswept, were huge plains and mountain ranges and salt flats—either bitterly cold or raging w
ith heat. Mongolia, the northern Chinese plains, the Bolivian altiplano, the Arizona desert—all of these look today as the corner of Pangea that would ultimately form Britain must have looked in Permian times: hot, dry, and very stable.
Except, with terrifying suddenness, another series of massive convulsions broke out. The oceans roared back in to inundate the plains. It must have been the almightiest of spectacles—much as if the South Pacific were to rise today, to flood all of China except its highest peaks, and then to lap hungrily away at and eventually flood to great depth the immensity of the Gobi Desert. And with the water came life, which burst forth once more and was soon teeming in wild profusion.
The warm oceans were benign and fertile again, and filled with living creatures—living marine beasts that, much more significantly for today’s geologists, died. Over the millennia the skeletons of a thousand species of their occupants, ammonites to belemnites, brachiopods to oysters, ichthyosaurs, and plesiosaurs, rained down to lie in vast thicknesses on the ocean floor below. In the jungles there were dinosaurs, as well as scores of types of early birds and dozens of strange creatures that had evidently crawled up from the ocean and out of the freshwater ooze, and that had readily joined in the battles for territorial supremacy.
The record of that sudden new explosion of life remains today firmly locked in the limestones and shales and sandstones of the Jurassic. Fossils are everywhere in the thick masses of Jurassic strata, attesting to an abundance that is the very opposite of the lifeless tedium of the Permian. And no matter that the name Jurassic itself comes from the Jura Mountains of Central Europe: it is an unequivocal reality that the best expression of the outcrop of the type rocks of this period is in the long yellow swath that William Smith confidently painted running southwest to northeast, right across the center of England.*
The conditions were not exactly similar during the millions of years of England’s Jurassic. The seas were shallow and warm only across the very center of England. Elsewhere the presence of huge landmasses made critical differences. Two of these landmasses above all help shape the outcrop that William Smith mapped—and across which I made my short pilgrimage.
The first, to the north, was a giant body of land called the North Sea Dome, which extended eastward to Russia and beyond, but in the west had spurs of high hills that ran across into Scotland and along the spine of what are now the Pennines. (No Jurassic rocks, of course, were laid down here, since no sea was present in which the sediment might be suspended.) To the south was the second, a low island fifty miles wide and oval shaped, extending over what is now the Brittany peninsula. The coastal seas that lay directly off these two landmasses were, as one might suppose, muddier, less rich in oxygen and very much shallower than those further away—and the Jurassic rocks that were laid down in them, and in the estuaries and sand flats that abutted them, were much less likely to be the thick-bedded limestones characteristic of oceanic conditions, and much more likely to be thinner and alternating layers of clay-silt-sand-limestone, clay-silt-sand-limestone that are the inevitable consequences of the ebbs and flows of the oceanic edge.
Which is precisely the case in South Dorset. As I wound my way slowly northward from West Bay, the rocks that the fossil record insisted were of Middle Jurassic age were a thin and confused mess of sediments. Evidently they had been deposited in shallow seas and estuaries, in basins that had been subjected to frequent earthquakes and faults, uplifts and downthrusts. And in all cases the outcrops displayed the so-called rhythms of the cyclothems—with clays deposited where the waters had been deep; then bluish or yellowish sands, as the waters began to recede; then limestones, becoming progressively finer-and finer-grained as the waters reached the climax of the ebb, and deposition came to a virtual end. After which the waters came flooding back, and with them the clays, the silts, and, as the shallowing began all over again, the sands and the limestones once more. Cyclothems are a feature of edge-of-ocean sediments: Dorset has them in abundance.
Along the outcrop the villages were adorned with stunningly pretty names—Salwayash, Bradpole, Beaminster, Melplash, Haslebury Plucknett, Ryme Intrinseca, Chedington, East Coker, Mosterton, and Netherbury. The countryside here was intimate, small scale, constantly changing—the softer limestones had been eaten by the acid rains and formed valleys, there were copses of pines growing in the sandy soil where caps of sandstones overlay and protected mounds of shale, and roads passed deep into gorges of soft brown sand that came away if you scratched the walls with a fingernail.
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,* 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.