by Ted Nield
When Pratt continued correcting readings taken in places near to the coast, the reverse was true. The ocean, despite its thick covering of less dense water, seemed to be pulling the plumb bob much more than it should have done. Pratt and the mapmakers were on the verge of one of the most fruitful discoveries in all geology. The Archdeacon wrote a paper for the Royal Society.
One of the things that makes science scientific is the fact that reputable journals will not publish anything before receiving the comments of one or more of the expert referees to whom they send every paper. This peer review remains a cornerstone of reliable science. It fell to George Biddell Airy, the Astronomer Royal, to review Pratt’s paper for the Royal Society. And it was he who came up with the geologically more correct explanation of these puzzling gravity anomalies. Mountains, Airy said, exert less gravitational pull than they should do because they have roots. Their less dense material extends down into the planet, in whose denser interior they float like icebergs in water. Continental masses, Airy said, stand high above the ocean floor because they are buoyant; in their case, floating in a substrate of denser rock. They stand proud, but only because they have much larger roots below. Mountains are higher than plains for the same reason that big icebergs stand taller than small ones.
A cartoon showing how, according to the Airy hypothesis of isostasy, like blocks of wood floating in water, mountains stand high because they are thick. Their light ‘roots’ lie immersed in the more dense rock of the mantle, like the counterbalancing nine tenths of an iceberg below the waterline
The ocean floor, on the other hand, is made of denser rock. To change the analogy from ice to wood, if continents are light, like balsa wood, and stand high in the water, the ocean floor is like mahogany or teak: so dense that it floats, but only just. Hence, despite all that water on top of them, the oceans still exert more gravitational attraction than scientists had expected.
FIGURE 1.-Generalized Profile, showing relative Areas of the Earth's Surface at different Heights and Depths.
The original ‘hypsographic curve’ by Grove Karl Gilbert, showing that the surface of the Earth lies mostly at two levels – average continent and average ocean. Mountains (extreme left) and ocean trenches (extreme right) account for very little of the Earth’s total surface area. Gilbert deduced that this graph said something fundamental about the way in which the Earth’s crust was organized. Reproduced by permission of the Geological Society of America, Boulder, Colorado © 1893.
Later it was discovered that if you make a graph of the Earth’s crustal elevation against the total area lying at that level, on this broad scale (at which small ups and downs can be neglected) the crust has only two basic levels. Continents are almost everywhere a few hundred metres above present sea level, and ocean basins are almost everywhere four to five kilometres below it. The continents have the odd mountain that’s very high and the oceans have the odd trench that’s very deep. But, basically, nearly all land is at one level and nearly all ocean floor is at another.
This is because ocean crust has its characteristic density and is the same everywhere (basalt), while continental crust is lighter and sits higher. And finally, there’s just enough water in the ocean basins to fill them, so nearly all continent is also land and nearly all ocean floor is under several kilometres of water.
This principle is called isostasy, but it is really no more than Archimedes’ Principle applied to rocks, which, contrary to all intuition, are all floating. Continents, despite what everyone thought they knew, and despite all the legends and myths, simply cannot sink. True, if you freight the land with thick ice sheets, then the extra mass of ice will gradually cause material underneath slowly to flow away. But when the ice melts, the deep, hot rock will flow back, and the land will rise again.
Although it took time, the idea of isostasy, of the buoyant balance of light and dense rock types, and the knowledge that, given time, the Earth is indeed soft to the touch, was what ultimately paved the way for a true understanding of how supercontinents form and disperse. They do it by moving sideways.
Old world, new world
In 1896, acknowledging the Wollaston Medal, the Geological Society of London’s highest honour, the sixty-five-year-old Eduard Suess wrote that it came to him ‘at an age when the natural diminution of physical strength confines me to my valley and my home; hammer and belt rest on their peg, and dreams and remembrances alone still carry me along those Alpine wanderings which form the highest charm of our incomparable science, and in the lonely grandeur of which Man feels himself more than ever a child of surrounding Nature’. His last years, devoted to writing his memoirs, were, according to the accounts, peaceful and happy.
Suess’s great book was completed nine years after he had retired from the university. His faithful servant, who had continued to bring and take away armfuls of books each day, quite unaware that his master had discovered a continent vaster than any man would ever see, looked at the foot-thick tomes, shook his head and said: ‘Is that all you managed to get out of those books I brought you?’
As a boy I inherited my father’s partwork encyclopaedia dating from about 1935, entitled The World of Wonder. This unwieldy tome contained popular and improving science and engineering articles. Edited by one Charles Ray, with copious but dingy black-and-white pictures and diagrams, it came in regular sections like ‘The Romance of Engineering’ or ‘Wonders of Land and Water’, punctuated by illustrated features with headings like ‘Inside a great Scotch boiler’ and my particular favourite, all about resonance, entitled ‘How a small girl can play a big trumpet’.
In year one of its publication, on page 172 there appears a short caption, set beneath three pictures: the Earth with its mountain ranges, a drying apple and an old man’s hand. It reads: ‘As the Earth gets older, its face wrinkles more and more, just as an apple wrinkles when it becomes dry and shrinks … and as the human skin wrinkles when a person becomes aged. Men of science are not agreed as to the cause of the Earth’s wrinkling. To some extent it is due to the shrinking of the Earth owing to the loss of interior heat …’
Suess’s cooling, shrinking Earth appeared to be still alive and well in the 1930s. But change was afoot. Turning to the World of Wonder’s page 731 (year two) you find another story, headed ‘The Drifting of the Earth’s Continents’. Three drawings show the world today, ‘3,000,000 years ago’ and ‘as it possibly appeared 200,000,000 years ago’: all lands locked together in one outline, one supercontinent, which we call Pangaea, and whose southern lobe bears the legend … ‘Gondwanaland’.
The iconic diagram by Alfred Wegener, showing Pangaea (consisting of Laurasia in the North and Gondwanaland in the South) slowly splitting up to form the continents we recognize today.
PART TWO
EXISTENCE OF LAW
6
WONDERLAND
Geologist finds hole that lured Alice
THE TIMES, 15 SEPTEMBER 1999
Down the rabbit hole
‘Down, down, down. Would the fall never come to an end? “I wonder how many miles I’ve fallen by this time?” she said aloud. “I must be getting somewhere near the centre of the earth. Let me see: that would be four thousand miles down, I think—”’
Lewis Carroll’s Alice’s Adventures in Wonderland (1865) was originally called Alice’s Adventures Underground, and its opening chapter makes it easy to see why. But the geological connections of this tale may go deeper than Alice’s (fairly accurate) reflections on the radius of the globe.
Lewis Carroll was an Oxford University mathematics don, whose real name was Charles Lutwidge Dodgson. He was born in Daresbury, Cheshire, in 1832, the son of a local curate. The family later moved across the Pennines to Yorkshire when Carroll’s father became a Canon of Ripon Cathedral. In that cathedral town Charles senior had a friend and colleague named Canon Baynes Badcock. Badcock was the Principal of Ripon College, a Church of England teacher-training school established in 1862. He lived in a house called Ure Lodge,
which took its name from the river that flows through this prosperous market town.
The Badcock family included a young daughter by the name of Mary: the model, many believe, for Sir John Tenniel’s illustrations of Alice when Wonderland came to be published. Lewis Carroll, a prolific pioneering photographer, is said to have passed her picture to the artist. However, amid this Trollopean idyll of rural canons, Oxford dons and genteel pursuits, dark forces were at work.
The district around Ripon is renowned for an alarming geological phenomenon: the sudden appearance of deep, vertical pits, often ten metres or more across, that can swallow trees, gardens, garages and even homes in seconds. These holes are not usually much deeper than twenty metres (though that is deep enough when the remains of your house are at the bottom of it). Worse, though many are dry, some are filled with churning water. As landscape features they are unstable. Left to their own devices, they gradually widen out as their walls collapse, evolving into shallow, conical depressions that may become ponds, marshes or empty hollows. Then they tend to be forgotten.
The fields opposite Canon Badcock’s home displayed many examples of this phenomenon, both old and new. In 1837 a particularly big hole developed: one the young Carroll may have seen, either on a visit, or when his family moved to Yorkshire in autumn 1843.
The annals of the Diocese of Ripon record similar collapses down to the present day. And in 1997 local and national newspapers reported that on the 23rd and 24th of April a large crater opened up outside the house of Mrs Jane Sherwood-Britton in Ure Bank Terrace, within the garden of Canon Badcock’s former home. The great maw quickly swallowed up two out of a row of four garages, and narrowly missed Mrs Sherwood-Britton’s two children, who had been playing beside them only minutes before.
If you visit Ripon you will notice that on the parapet of the Town Hall is inscribed the pious motto ‘EXCEPT YE LORD KEEP YE CITTIE YE WAKEMAN WAKETH IN VAIN’. The annually elected Ripon Wakeman is an ancient office that may date back to Saxon times, though the earliest firm evidence is from 1515. In 1604, with the coming of a new City Charter, the Wakeman was restyled Mayor; but his original job was to keep the peace and act as watchman from the blowing of the Horn, in the Market Place at 9pm, until dawn. (The Hornblower, incidentally, still does his stuff today.)
The Lord may keep the city of Ripon, but to any inhabitant unfortunate enough to suffer from one of its occasional subsidences, the Lord is not much help. English law is distinctly quirky about landslip and subsidence, for which it holds landowners responsible, even though, you might think, such natural phenomena fit the bill perfectly as ‘acts of God’. As far as subsidence is concerned, landowners’ faith is better placed in geologists than the Almighty.
Nature rarely respects property boundaries, and legal argument at Ure Bank Terrace has since concentrated on demonstrating that no single proprietor can be held responsible for the whole of the hole, two of the four garages that vanished having belonged to two other people, one the modern owner of Canon Badcock’s Ure Lodge. And, bringing the story full circle, while the authorities were trying to find that elusive person (who has still to be identified), Ure Lodge itself also succumbed to local geology. Abandoned, empty and crumbling, it was finally transformed by workmen, who took it to pieces and recycled it.
The alarming tendency for bits of Ripon to vanish without warning down deep holes creates many civil engineering problems requiring close cooperation between engineers and geologists. Ripon now boasts a bypass, much of which is elevated above the River Ure on a reinforced embankment. The river flyover had to be designed so that it would remain standing should the ground suddenly vanish from beneath any one of its many well-founded piers. This is comforting to the motorist, because holes like the one that almost swallowed Mrs Sherwood-Britton’s house appear on average once every three years, and (on a human timescale) are never going to stop.
In fact, if you tot up the combined volume of the thirty-five subsidence hollows that have formed in the past century and a half, it comes to at least 27,000 cubic metres, according to geologist Dr Tony Cooper. Cooper, an expert on the geology of the area, calculates that under each square kilometre of the Ripon district, at least thirty-six cubic metres of rock are dissolved every year by groundwater.
Tony Cooper recently retired as District Geologist for Yorkshire at the British Geological Survey, where he worked for thirty years. There is a boyish candour and enthusiasm about the way he talks about these alarming subsidences, and an obvious glee when, on his computer, he overlays digitized versions of historic maps one after the other to show how, over the decades, cartographers have mapped ‘pits’ that in later years became shallow depressions and then boggy hollows. Then there’s one map that shows a building on the site, the old pit now filled in and long forgotten. The modern technical sophistication of the Survey’s digital resources is a far cry, however, from the hammer, compass-clinometer, pencil and field-slip world of geology in the 1970s.
‘When I first joined the Survey I started mapping in Yorkshire,’ Cooper told me, ‘near York, then Wetherby, Knaresborough, Ripon and Bedale. I started to come across holes in the ground that I couldn’t explain. The first were south of Knaresborough. On the floodplain of the river, in a dead straight line, I found four craters. They were fairly sharp-sided, but they had no spoil heaps, so they didn’t look like someone had dug them. As I worked north towards Ripon I came across more, and started delving into the old literature. By 1982 I had sorted out the story, and come across quite a lot of other historical material’ – including one Revd Tute and the paper ‘On certain natural pits in the neighbourhood of Ripon’ that he presented to the seventy-third meeting of the Geological and Polytechnic Society of the West Riding of Yorkshire in the Wakefield Church Institute one April night in 1869.
From the Revd Tute’s first steps towards a rational understanding of Ripon’s subsidence problems comes our modern knowledge that Ripon’s bedrock conceals layers of calcium sulphate – the soluble mineral known as gypsum (raw material for plaster) – in two thick strata.
Tony Cooper saw that prominent joints in the rock – natural partings and lines of weakness – were dictating the sites where dissolution of this gypsum was having the greatest effect. Rocks rarely have just one set of joints; they tend to occur in what are called conjugate sets, rather like the intersecting cardboard sheets in a case of wine. Where one set of joints intersects another, Cooper realized, dissolution was doubly enhanced. That was where the biggest caves formed, so giving rise to the regular, lattice-like pattern of sinkholes dotted about the landscape.
But Cooper was also responsible for another discovery, the link between Ripon’s unique geology and the world’s most famous children’s book. Lewis Carroll’s connection with Ripon was already well known: some of his characters were allegedly inspired by carvings on misericords in the Cathedral’s choir stalls. If you inspect the misericord under the Mayor’s seat (north side, east end) you will see a gryphon chasing two rabbits, an allegory of evil preying on the weak. One rabbit has been caught, but one has dived into its hole – you can see his little scut just disappearing into the Earth. Gradually, as Cooper pored over the old geological literature, he began to notice a recurring coincidence of collapses, clergymen and Lewis Carroll.
By 1997 Cooper’s notion about the link between Carroll’s imaginative landscape and the landscapes the writer had known as a boy was growing stronger. He began reading up on the author’s life and while doing so he also noted an earlier, possibly significant geological association, with the village of Croft-on-Tees, south of Darlington, where the Dodgsons moved when Carroll was eleven years old. Barely a mile from where they lived are some mysterious (reputedly bottomless) ponds called Hell Kettles, one of which was formed by a dramatic subsidence in 1179. To this day, Hell Kettles are filled with water that rarely freezes, visibly steaming in the winter, and giving off sulphurous smells. As at Ripon, the subsidence was caused by gypsum dissolving underground. Such a local
landmark and its diabolical legend could not have failed to come to the notice of the inquisitive young writer.
Cooper, who like Carroll is an enthusiastic amateur photographer, carries his enthusiasm for geology and landscape into many other fields, and this excursion into literary research was for him not that unusual. But his theory lacked an outlet until 1999, when the annual UK science festival organized by the British Association for the Advancement of Science rolled into Cooper’s alma mater, Sheffield University.
The tide of ideas that had been rising in Cooper’s mind for the best part of a decade had suddenly found an outlet. Sheffield University geologist Dr Mike Romano was charged with organizing the geology sessions and contacted him to speak about the recent Ripon subsidence. Cooper decided to enliven his story about gypsum dissolution by linking it to something everybody knew about: Alice in Wonderland. The rest is a small part of media history.
In fact, everything about Ripon’s civil engineering problems is history, except the modern technology to deal with them. The collapses happen because groundwater dissolves gypsum lying beneath the whole region. As caverns open up within it deep underground, their roofs begin to spall and stope their way slowly upward to the ground surface. If you have ever removed a fireplace and then watched in horror as the bricks in the chimney-breast above fall out on to the floor, course by course, you have seen this process at work.