2
The Living Earth

From his early youth, Leonardo was fascinated by pinnacles of rocks, carved out by water and eventually turning into gravel and fertile soil, and he came to see water as the chief agent in the formation of the Earth’s surface. As a young boy, he explored the rocky outcroppings, waterfalls, and caves in the Tuscan countryside around Vinci. Later on, they became the defining elements of his personal mythical landscape—the fantastic rock formations that would forever appear in the shadowy backgrounds of his paintings.

Leonardo’s keen awareness of the continual interaction of water and rocks motivated him to undertake extensive studies in geology. During his travels in central and northern Italy as a military and hydraulic engineer, he studied the erosion of rocks, deposits of gravel and sand, and strata of sedimentation and produced many detailed maps of the regions he visited. His geological observations, like those in all the other branches of his science, display an astonishing accuracy. On the basis of these extensive and methodical analyses, he formulated a series of geological principles with a clarity that would not be achieved again until the twentieth century—principles that are still taught in geology courses today.¹

The Bones, Flesh, and Blood of the Earth

Leonardo’s expositions of geology, and especially his ideas about the formation of the Earth, sound so modern because, unlike most of his contemporaries, he was fully aware of the long duration of geological time, and, like geologists today, he viewed the Earth as a dynamic and continually changing entity. The philosophical basis of this conception was the idea—central to Leonardo’s science—that the Earth as a whole is alive and that the patterns and processes in the microcosm, the human body, are similar to those in the macrocosm, the body of the living Earth.

FACING Horizontal outcrop of rock, c. 1510–13 (detail, see fig. 2-4).

As in many of his scientific investigations, Leonardo begins by summarizing this ancient doctrine. The earliest of his summaries is found in Manuscript A, written around 1490–92, during his first period in Milan:

Man has been called by the ancients a lesser world, and certainly the term is well applied; because, since man is composed of earth, water, air, and fire, this body of the Earth is similar. If man has within himself bones as support and framework for the flesh, the world has the rocks as support of the earth. As man has within himself the pool of blood where the lungs increase and decrease with the breathing, so the body of the Earth has its ocean tide, which also increases and decreases every six hours with the breathing of the world. As from the said pool of blood originate the veins that spread their branches through the human body, so the ocean fills the body of the Earth with infinite veins of water.²

After this summary of the views of the ancients, Leonardo immediately notes that there is also an important difference between the body of the Earth and the human body: the Earth has no tendons. He argues that there is no need for tendons in the Earth, because the Earth (according to Aristotelian cosmology) does not move:

In the body of the Earth, the tendons are lacking, and these are not there because tendons are made for the purpose of movement. And as the world is perpetually stable, no movement takes place; and since no movement takes place, the tendons are not necessary. But in all other things [man and the Earth] are very similar.³

Eighteen years later, Leonardo restates the idea of the living Earth in a more elaborate way. Whereas in his earlier statement, the analogy between the human body and the body of the Earth was based on the Aristotelian notion that both are composed of the four elements, he now goes beyond the level of a mere analogy. He justifies the conception of the Earth being alive by observing that the processes of growth and renewal, which are common to all life, are pervasive on Earth:

Feathers grow on birds and change every year; hairs grow on animals and every year they change, except in some parts, like the hairs of the beards of lions, cats, and the like. Grass grows in the fields and leaves on the trees, and every year they largely renew themselves.⁴

He concludes from this observation that the Earth must be endowed with a vital force, or soul,* which generates forms and processes that are similar to those in the human body:

We may therefore say that the Earth has a vital force of growth, and that its flesh is the soil; its bones are the successive strata of the rocks which form the mountains; its cartilage is the porous rock, its blood the veins of the waters. The lake of blood that lies around the heart is the ocean. Its breathing is the increase and decrease of the blood in the pulses, just as in the Earth it is the ebb and flow of the sea.⁵

As noted earlier, late in his life Leonardo abandoned the analogy between the blood vessels of the human body and the water veins of the Earth as being too restricted; he realized that the pathways of the water cycle in the macrocosm and the blood cycle in the microcosm, as well as the forces driving them, are quite different (see p. 29). However, he always maintained the basic conception of the living Earth. Whenever he explored the forms of nature in the macrocosm, he also looked for similarities of patterns and processes in the human body. In doing so, he went beyond the general analogies that were common in his time and drew parallels between very sophisticated observations in both realms. For example, as I shall discuss in subsequent chapters, he used his knowledge of turbulent flows of water to understand subtle details of the movement of blood in the heart and aorta (see pp. 300ff.). He saw the “vital sap” of plants as their essential life fluid and observed that sap nourishes the plant tissues as blood nourishes the tissues of the human body (see pp. 117–18). He took these observations as compelling testimonies to the unity of life at all scales of nature.

Leonardo’s conception of the Earth as being alive, manifesting patterns and processes common to all living systems, was a forerunner of the modern Gaia theory, which views our planet as a living, self-organizing, and self-regulating system.⁶ In fact, the originator of the contemporary theory, atmospheric chemist James Lovelock, even used analogies in one of his books that are somewhat similar to Leonardo’s, except that Lovelock compared the Earth to a giant redwood tree rather than to the human body.⁷

Leonardo’s views of rocks, soil, and water as the bones, flesh, and blood of the living Earth was the philosophical and perhaps even spiritual foundation of his lifelong fascination with dramatic rock formations, shaped by the action of water. He not only analyzed in his scientific writings how the body of the Earth is continually transformed by the interplay of erosion and sedimentation but also pictured these processes in many of his paintings and drawings with astonishing accuracy and persuasive power. “No artist, before or since,” writes art historian Martin Kemp, “has quite equaled the suggestive magic with which [Leonardo] insinuates his vision of the inner and outer oneness of created forms into drawings and paintings.”⁸

All of Leonardo’s science is utterly dynamic. He portrays nature’s forms in ceaseless movement and transformation, recognizing that living forms are continually shaped and transformed by underlying processes. This dynamic conception of nature is evident in his studies of anatomy, botany, and fluid dynamics, and it is perhaps most striking in his geology. In Leonardo’s view, the Earth, being a living body, is continually shaped and transformed over long periods of time. This is why his geological concepts sound so modern to us.

In the introductory chapter of a geology textbook published in 1995, we find the following passage.

The Earth is a dynamic planet that has continuously changed during its 4.6-billion-year existence…. We can easily visualize how mountains and hills are worn down by erosion and how landscapes are changed by the forces of wind, water, and ice. Volcanic eruptions and earthquakes reveal an active interior, and folded and fractured rocks indicate the tremendous power of the Earth’s internal forces.⁹

This passage, except for a couple of modern terms, would not look out of place in Leonardo’s Notebooks.

The Sculpting of Mountains and Valleys

Like other hydraulic engineers in the Renaissance, Leonardo was very familiar with the erosion of river banks, especially during the periodic flooding of Alpine rivers in the Lombard region (see p. 32). His Notebooks, notably the Codex Leicester, contain numerous suggestions for dealing with these practical problems. Unlike his fellow engineers, however, Leonardo went beyond empirical rules, integrating his acute observations into a far-reaching theory of how water, over long periods of time, has sculpted mountains and valleys on the surface of the Earth.

To begin with, Leonardo explains that erosion results from the friction between water and earth: “The actual rivers have a clouded flow because of the earth that rises in them as a result of the friction of their waters on the bed and the banks.”¹⁰ Then he describes in great detail how this process of erosion creates entire valleys over time:

Although it is almost level, many rivulets will originate in the lowest parts of the surface, and these will begin to hollow out and form receptacles for other surrounding waters. In this way, every part of their course will become wider and deeper, their waters steadily increasing, until all this water will drain away. And these concavities will become the courses of the torrents that receive water from the rains, and thus the banks of these rivers will continue to erode until the spaces in between become steep hills.¹¹

From these observations, Leonardo concludes that all valleys are created by flowing water: “Each valley has been created by its river, and there is the same proportion between valleys as between rivers.”¹² Remarkably, he states that this assertion can be verified by correlating layers of rocks on the two sides of a valley. He argues that the occurrence of the same sequence of superimposed rock strata on opposite slopes of a valley is the clearest proof of the fact that valleys were created by water cutting into high mountains:

The rivers have all sawn through and divided the members of the great Alps one from another; and this is manifest from the order of the layered rocks in which, from the summit of the mountain down to the river, one sees the strata on one side of the river corresponding with those on the other.¹³

With this argument, Leonardo was two hundred years ahead of his time. The superposition of rock strata would not be recognized and studied in similar detail until the second half of the seventeenth century.*

In his careful studies of erosion, Leonardo gave detailed descriptions of the transport and deposition of rock fragments by rivers and streams. In particular, he noted the sequence of what a river deposits as it flows from the high mountains down to the river’s mouth:

The river that flows out from the mountains deposits a great quantity of large stones in its gravelly bed, and these stones still retain some of their angles and sides; and as it proceeds on its course it carries with it smaller stones with angles more worn away, and so the large stones become smaller; and farther down it deposits coarse and then minute gravel, … and after this follows sand, at first coarse and then fine, and then coarse and fine mud … and then the mud becomes so fine that it seems almost like water … and this is the white earth that is used for making jugs.¹⁴

Today, this sequence of processes—known as rounding, sorting, and sedimentation—is recognized as a basic principle of geology.† “I learned this rule,” remembered evolutionary biologist Stephen Jay Gould, “as principle number one on day number one in my college course in beginning geology.”¹⁵

Like modern geologists, Leonardo viewed the erosion, transformation, and sedimentation of rock fragments as one single process through which water continually shapes the surface of the Earth. “Water wears away the mountains and fills up the valleys,” he wrote in the Codex Atlanticus. “If it could, it would like to reduce the Earth to a perfect sphere.”¹⁶ He carried out detailed studies of the shapes and dynamics of rivers, the hollowing out and silting of their beds, and the formation and development of meanders. In addition, he produced many beautiful and accurate maps of watersheds and lakes.

FIG. 2-1. Topographical map of the region northwest of Florence, showing Lucca, Pistoia, and Prato in the north and the Arno valley in the south, as well as the trajectory of the canal envisaged by Leonardo, c. 1503–4. Windsor Collection, Drawings and Miscellaneous Papers, RL 12685r.

Leonardo’s Maps

Even the casual viewer cannot fail to notice the surprisingly modern look of Leonardo’s geographical maps. He used cartographic techniques that surpassed anything attempted by medieval and Renaissance mapmakers. His maps often show distances based on elaborate odometer readings, involving ingenious instruments he himself had designed;¹⁷ elevations are indicated by washes of different colors and shades, as in our modern atlases; and in some maps he uses a special relief technique to create “aerial views” of the depicted landscapes (figs. 2-1 and 2-2). In these beautiful and very detailed topographical renderings, the rivers, valleys, and settlements are pictured in such a realistic manner that one can have the eerie feeling of looking at the landscape from an airplane.

In most of his maps, Leonardo focused specifically on networks of rivers and lakes, often because he contemplated some hydraulic project in that region. For example, several detailed maps of the Arno basin (for example, fig. 2-1) were produced in connection with his plan of building a giant canal to bypass the river Arno.¹⁸ Similarly, the beautiful large map of the Chiana Valley (fig. 2-2) may have served to illustrate the idea to use Lake Trasimene, pictured prominently in the map, as a source of water for the Arno during the dry summer months.

FIG. 2-2. Bird’s-eye view of the Chiana Valley, showing the cities Arezzo, Perugia, Cortona, and Siena, c. 1502. Windsor Collection, Drawings and Miscellaneous Papers, RL 12278r.

FIG. 2-3. Map of Imola, 1502. Windsor Collection, Drawings and Miscellaneous Papers, RL 12284.

A curious feature of this map is the fact that Leonardo has severely distorted the relative scales of its center and periphery in order to fit the outlying parts into the given format, while correctly picturing the continuity of the terrain and its intricate waterways. I have argued that this may be viewed as an example of the “topological” thinking that is also evident in many of Leonardo’s architectural studies and mathematical diagrams.¹⁹

Because they served as supporting documents for military strategies or projects of hydraulic engineering, Leonardo’s maps often had to be very precise and accurate, but many are also works of art that depict human settlements and built structures within their natural environment. In these drawings, the watercourses and geological features embody the vitality and continual movement of the Earth’s living forms. This strong sense of the living Earth is conveyed even in some of Leonardo’s maps of cities. The most famous is a very detailed map of Imola, a small town in Emilia-Romagna where Leonardo spent several months during the winter of 1502 as military engineer in the service of Cesare Borgia.²⁰ He designed new fortifications for the town’s citadel and drew a highly original circular map (fig. 2-3), in which the main streets, town walls, and strategic routes are pictured with great precision, along with various bearings and distances to other cities.

What makes the map of Imola so remarkable and so beautiful is Leonardo’s artistic choice of placing the precise “aerial” view of the fortified town within a natural environment of fields, paths, and various dwellings near the town, including in particular the sweeping meanders of the river Santerno. In the words of art historian Daniel Arasse,

This map, a real synthesis of science and art, reflects the feeling that Leonardo has for the natural world, and for the way, simultaneously glorious and fragile, human activity expresses its rationality in it. While the survey of the housing of Imola impresses by the elegance of its geometry, the striking curves made by the flow of the river Santerno reveal, like the vein-like tributaries of the rivers and streams of central Italy, the feeling for the “life of the earth,” this organism, this living macrocosm.²¹

Geological Time

Leonardo carried out careful and detailed studies not only of erosion but also of the corresponding process of sedimentation—the deposition of suspended rock fragments at the mouths of rivers and in the ocean. He recognized that sedimentation often occurs in successive layers:

The stratified rocks of the mountains are all in layers of mud deposited one above another by the floods of the rivers…. The different thicknesses of the strata of the rocks are created by the different floods of the rivers, that is the greater and the lesser floods.²²

In particular, Leonardo discerned a specific sedimentary structure known to modern geologists as “graded bedding,” in which the grain size of the sediments decreases upward within a single layer, or “bed,” and he correctly explained its formation:

Each layer is composed of heavier and lighter parts; the lowest being the heaviest. And the reason for this is that these layers are formed by the sediments from the waters discharged into the sea by the current of the rivers that flow into it. The heaviest part of this sediment was the part that was discharged first in the sequence.²³

In his drawings and paintings, Leonardo pictured many geological structures with extraordinary accuracy. In his masterwork Madonna and Child with Saint Anne, a beautiful example of graded bedding is shown at the feet of Saint Anne (plate 7). The decrease of grain size from the bottom of the bed to the top is clearly visible.

Leonardo’s recognition of temporal sequences in the strata of soil and rock and of the way in which valleys are created by flowing water led him to a momentous conclusion: that the forms of the Earth are the result of slow processes taking place over long epochs of what we now call geological time. “Since things are far more ancient than letters,”* he wrote in the Codex Leicester,

it is not to be wondered at if in our days no record exists of how these seas extended over so many countries…. But for us it is sufficient to have the testimony of things born in the salt waters and found again in the high mountains, sometimes far away from the seas.²⁴

FIG. 2-4. Horizontal outcrop of rock, c. 1510–13. Windsor Collection, Landscapes, Plants, and Water Studies, folio 53r.

With this view, Leonardo was centuries ahead of his time. Geologists became aware of the great duration of geological time only in the early nineteenth century with the work of Charles Lyell, who is often considered the father of modern geology.²⁵ Leonardo fully realized the importance of the understanding of geological time for scientific knowledge as a whole. “Knowledge of the past and of the surface of the Earth is ornament and food for human minds,” he wrote in the Codex Atlanticus.²⁶

Leonardo was also the first to introduce the notion of folds of rock strata (piegamenti delle falde delle pietre).²⁷ His ideas of how rocks are formed over enormously long periods of time in layers of sedimentation and subsequently shaped and folded by powerful geological forces come close to an evolutionary perspective. Leonardo arrived at this perspective three hundred years before Charles Darwin, who also found inspiration for evolutionary thought in geology, in particular in the works of Lyell.²⁸

The Windsor Collection contains a drawing of a horizontal outcrop of rock (fig. 2-4) that illustrates Leonardo’s conception of evolutionary geological processes in dramatic fashion. It shows horizontal strata of sedimentary rock that have been severely eroded and warped. “This drawing clearly manifests the shaping hand of time,” writes Kemp, “and bears witness to the great forces that have molded the strata into their contorted configurations.”²⁹

FIG. 2-5. A stream running through a rocky ravine, c. 1483. Windsor Collection, Landscapes, Plants, and Water Studies, folio 3r.

One geological formation that fascinated Leonardo throughout his life was the pinnacle of jagged rocks that appears so often in the backgrounds of his paintings. These rocky towers are remnants of long erosional processes that have stripped away softer layers of rock and exposed harder, more weather-resistant rock underneath. These processes of weathering and erosion are shown clearly in the very first of Leonardo’s drawings of rock formations, a picture of a rocky ravine with water birds produced around 1483, shortly after his arrival in Milan (fig. 2-5).

The dominant feature of the drawing is a cliff that has lost almost all its soil and vegetation except for a few twisted trees that cling precariously to its rim. The entire rock has been split into large vertical blocks, a cluster of pinnacles, and boulders of ever-smaller sizes by the persistent action of weathering and water through the ages. As in the previous drawing, there is a palpable sense of continuous movement and transformation.

The Virgin of the Rocks

Leonardo’s most detailed and sophisticated depiction of rock formations is to be found in his early masterpiece, the Virgin of the Rocks, now in the Louvre (plate 8). The painting, created between 1483 and 1486, caused a sensation in Milan’s artistic and intellectual circles and marked the beginning of Leonardo’s great fame as a painter.³⁰ The work was unprecedented on many levels. It was revolutionary in its rendering of light and dark (chiaroscuro). Its low tones of olive green and gray were in stark contrast to the bright colors of Italian fifteenth-century art, and its composition represented a complex and controversial meditation on the destiny of Christ.³¹ In addition, the Virgin of the Rocks is testimony to Leonardo’s powers of scientific observation and profound knowledge, unmatched in his time, of geological formations and plant growth.

Geologist Ann Pizzorusso, who has carried out a detailed study of the painting’s geological features, has called it “a geological tour de force” because of “the subtlety with which Leonardo represents a complicated geological formation.”³² As Pizzorusso shows in her article, the different rocks in the grotto, which were unnamed when Leonardo studied and painted them, are rendered with such accuracy that they can be readily identified and described with proper technical terms by a modern geologist.

A key distinction made in geology today is that between sedimentary and igneous rocks. Sedimentary rocks form at or near the Earth’s surface by sedimentation and subsequent consolidation of rock fragments and dissolved minerals. Igneous rocks, by contrast, are formed within the Earth’s crust when liquid magma cools and crystallizes. Since molten rock is less dense than solid rock, it tends to move upward toward the surface. It may end up being ejected as lava by volcanoes, or it may intrude into layers of sedimentary rock below the surface and crystallize there into igneous rock. The fluid pressure of the intruding magma can be so great that it actually lifts the overlying rock layers.³³

From his extensive geological studies, Leonardo was familiar with the processes of sedimentation and compaction of fragments into sedimentary rock, but it is doubtful that he understood the origin of igneous rock. Although igneous rock can be seen after erosion has exposed it at the Earth’s surface, its formation within the crust can be studied only indirectly and presents a great challenge to geologists even today. Yet Leonardo was able, based on meticulous observations, to draw numerous fine details of the texture of rocks to such an extent that modern geologists can easily recognize telltale signs of the geological processes that formed.

In the Virgin of the Rocks, Leonardo pictured a complex geological formation that resulted from the intrusion of an extremely hard igneous rock, known as diabase, into soft layers of sandstone, one of the most common sedimentary rocks.* Both the sandstone and the diabase are weathered, and Leonardo has pictured accurately how the surfaces of the two types of rock have weathered differently in accordance with their respective hardness. “The result,” explains Pizzorusso, “is an accurate portrayal of weathered sandstone intersected by diabase, which is much more resistant and therefore retains its structural characteristics.”³⁴ Pizzorusso illustrates her analysis of the geology in the Virgin of the Rocks with a sketch of the painting’s principal features, to which she has added the technical terms for the rock formations shown (fig. 2-6).

At the top of the grotto, we see rounded (“spherically weathered”) mounds of sandstone, which have decomposed sufficiently to allow roots to take hold in them and plants to grow. Above the Virgin’s head, jutting out in vertical relief, is the igneous diabase, which had intruded into the layers of sandstone and lifted the top layer in the distant geological past, thus intersecting the two layers with a sheet of hard rock, or “sill.” As it cooled, the diabase contracted and produced vertical cracks with sharp edges, known as columnar fractures or joints.

FIG. 2-6. Geological features in Leonardo’s Virgin of the Rocks. From Pizzorusso, “Leonardo’s Geology.”

The layer of sandstone below the diabase shows the same rounded weathering pattern as the top layer, and the contact surfaces between the two types of rock are clearly indicated by horizontal lines. Leonardo also observed correctly that no vegetation is growing out of the diabase, which is far too hard and too resistant to erosion to provide support for plants.

In the foreground of the grotto, the sandstone is not heavily weathered and has thus retained its original structure. The typical horizontal layers, or “beds,” have been rendered with the utmost accuracy. In the distant background, finally, we can perceive the rocky pinnacles that appear in so many of Leonardo’s drawings and paintings. They are remnants of processes of erosion in the distant past that have stripped away softer layers of rock, exposing the harder, more weather-resistant rock underneath.

Leonardo produced two versions of the Virgin of the Rocks. The dates and circumstances of the second version, which now hangs in the National Gallery in London, are not quite clear, and art historians believe that Leonardo may have let a fellow painter, Ambrogio de Predis, execute large parts of it.³⁵ This seems to be confirmed by Ann Pizzorusso’s comparison of the geological details in both paintings. She finds that, even though the differences between the rock formations in the two versions may not be immediately obvious to the layperson, close evaluation shows that the rendering of geological details in the London version is clearly inferior to that in the Louvre.

“An observer with some knowledge of geology,” Pizzorusso writes, “would find that the rock formations represented in the National Gallery work do not correspond to nature.” With a detailed geological analysis, she demonstrates that the rocks in the London version are “synthetic, stilted characterizations,” and she concludes: “It seems unlikely that the same person could have portrayed geological formations so accurately in the Louvre work and so incongruously in the National Gallery painting.”³⁶ A similar conclusion was reached by botanist William Emboden on the basis of a comparison of botanical details in the two versions of the Virgin of the Rocks (see p. 105).³⁷

The Fossil Enigma

In his extensive studies of sedimentation, Leonardo did not fail to notice that the sedimentary rocks of the Apennine mountains in northern Italy contained numerous fossils, and he correctly recognized them as the traces of organisms that had lived in the distant past:

Between the various layers of the rocks are still to be found the tracks of the worms which crawled upon them when they were not yet dry, [and] all the marine clays still contain shells, and the shell is petrified together with the clay.³⁸

The existence of fossils had been known since antiquity. Several ancient and medieval authors had commented on them without being aware of their true nature and age. Leonardo was familiar with these classical and medieval texts,³⁹ and in addition he had plenty of opportunities to observe well-preserved specimens of fossilized plants and animals. In fact, he tells the story that large numbers of fossil shells in perfect condition were brought to him in Milan:

In the mountains of Parma and Piacenza multitudes of shells and corals filled with worm-holes may be seen still adhering to the rocks. When I was making the great horse at Milan a large sack of those, which had been found in those parts, was brought to my workshop by some peasants, and among them were many that had been conserved in their original condition.⁴⁰

Marine fossils represented an enigma that natural philosophers had debated intensely since antiquity. If fossil shells were indeed remnants of marine organisms, how did they end up in sedimentary strata that now lie in the high mountains, several thousand feet above the current sea level? Leonardo was keenly aware of the problem:

How will you explain the infinite number of kinds of leaves frozen into the high rocks of these mountains, among them seaweed that one finds mingled with shells and sand? And you will also see all kinds of petrified things, such as sea crabs, broken into fragments, scattered and interspersed with those shells.⁴¹

To solve the enigma, Leonardo studied a wide variety of body fossils (remains of organisms) and trace fossils (tracks of worms and snails) with the utmost care. On the basis of his detailed and sophisticated observations, he then presented his conclusions in a systematic exposition that covers several pages in the Codex Leicester and, with a series of lucid and original arguments, refutes the theories that were current in the Renaissance. Leonardo’s exposition is an example of brilliant scientific reasoning, resulting in the first correct explanation of the nature of fossils. This understanding would not appear again in science until the end of the eighteenth century.

During the Middle Ages and the Renaissance, the most common belief about the origin of fossil shells in the mountains was that they had been carried there from the sea on the high waters and strong currents of Noah’s flood, the Great Deluge of the Bible. A second, more esoteric theory was that these fossils were not remains of marine organisms but were special kinds of minerals that had been created right there in the mountains by divine intervention, or had grown in the rocks as a result of “celestial influences.” Leonardo looked on both of these theories with disdain. He wrote of “the stupidity and simple-mindedness of those who imagine that these creatures were carried to such places distant from the sea by the Deluge,” and “how another sect of ignoramuses maintain that nature or the heavens have created them in these places through celestial influences.”⁴²

In the Codex Leicester, Leonardo presents five powerful arguments against the diluvial theory. He notes that fossils appear in several layers of sedimentary rocks that were deposited at different sequential times, which is inconsistent with the biblical story of a single flood. “If you should wish to say that there were several deluges to produce these layers and the shells within them,” he argues, “you would also have to affirm that such a deluge took place every year.”⁴³ Moreover, he reasons, according to biblical tradition, the Deluge rose to “seven cubits above the highest mountain,” and hence the shells would have been deposited at the very top of these mountains. But instead they are found predominantly near the mountain bases, “and always at the same level, layer upon layer.”⁴⁴

In addition to these two basic points, Leonardo uses sophisticated observation as the basis for three further arguments. Fossil strata deposited by strong currents, he explains, would not have preserved the tracks and trails of marine organisms that he found in sedimentary rocks. To him, these tracks are important evidence for the fact that the marine creatures were alive in the environment in which their fossilization occurred, rather than being carried there as dead animals by the Deluge.

Moreover, “if the Deluge had to carry shells for distances of 300 and 400 miles from the sea, it would have carried them mixed with various other species, all heaped together.” But instead, “we see at such distances oysters all together, and the shellfish, and cuttlefish, and all the other shells that congregate together.”⁴⁵

Leonardo then notes that he has seen some fossils of bivalves (oysters, clams, and the like) with their two shells still joined. Since these shells are not cemented together in life but linked by an elastic ligament that quickly decays after death, any extensive transport in a flood would have separated the two valves. Hence, these animals must have been buried where they lived:

On the edges of the rocks, we find a few with paired shells, like those that were left behind by the sea, buried alive in the mud, which later dried up and, in time, became petrified.⁴⁶

Having established that the marine fossils could not have been transported to the mountains as dead animals by the currents of the Deluge, Leonardo then counters the argument that they might have migrated there through the high waters. He explains in great detail that the cockle, a bivalve mollusk often seen in fossils, could not have moved so far from the sea in the forty-day time period traditionally associated with the biblical Deluge:

The cockle is an animal of no more rapid movement than the snail, when out of water, and is even somewhat slower because it does not swim but makes a furrow in the sand; and, using this furrow to support itself, it will travel between 3 and 4 braccia* a day. Therefore this creature with such motion could not have traveled from the Adriatic sea as far as Monferrato in Lombardy, which is 250 miles away, in 40 days.⁴⁷

In the same pages of the Codex Leicester, Leonardo also dismisses the belief that fossils are special minerals that have grown in the mountains under the influence of celestial forces. He argues that such minerals would grow in all strata of rocks, not only in those showing clear evidence of marine origins. “Where the valleys have not been covered by the salt waters of the sea,” he points out, “there the shells are never found.”⁴⁸

In addition, Leonardo argues that the fossil shells must have been parts of living organisms in the past because they display features that are present in other life forms. In particular, he observes that the age of a fossil shell can often be determined from growth rings that record cycles of months and years:

In these places … we [can] count in the crusts of cockles and snails the years and months of their life, as we do in the horns of oxen and sheep, and in the branches of trees that have never been cut in any part.⁴⁹

This statement is a lucid example of the systemic thinking that is pervasive in Leonardo’s science. Understanding a phenomenon, for him, always meant connecting it with other phenomena through a similarity of patterns. That he was able to associate the annual rings in the branches of trees with the growth rings in the horns of sheep is remarkable enough. To use the same analysis to infer the lifespan of a fossilized shell is extraordinary. As biologist Stephen Jay Gould has pointed out, this analysis of periodicities in growth became a rigorous and important subject in paleobiology only in the twentieth century.⁵⁰

Implicit in all of Leonardo’s arguments is his attempt to explain the origin of fossils in terms of natural processes that can be observed in the present, rather than by some miraculous or catastrophic events in the past. This idea was formulated explicitly as the so-called uniformitarian principle by the Scottish geologist James Hutton in the eighteenth century and was firmly established in the nineteenth century by Charles Lyell. The twin ideas that past changes in the Earth’s surface can be understood in terms of forces and processes still operating today, and that these processes took place over extremely long periods of time, have since become cornerstones of modern geology. Both ideas were first set forth by Leonardo da Vinci and were integral to his science of the living Earth.

Leonardo’s explanations of the nature of fossils are accompanied by precise descriptions of the specific sites in which he observed these deposits. According to historian of geology François Ellenberger, “we are justified, without being anachronistic, to talk about Paleoecology…. A modern geologist can only express surprise when he reads these texts in which there is nothing either erroneous or of no utility.”⁵¹

In addition to recording the fossils’ depositional environments, as they are called by geologists today, Leonardo describes the process of fossilization itself in great detail. “As befitted a sculptor who was urgently concerned with bronze casting,” writes Martin Kemp, “Leonardo provided wonderfully vivid reconstructions of the casting of shells, exoskeletons and whole organisms within their moulds of enveloping mud.”⁵²

“When nature is on the point of creating stones,” Leonardo explains, “it produces a kind of sticky paste which, as it dries, congeals into a solid mass together with anything it has enclosed in itself. However, it does not change it into stone but preserves it within itself in the form in which it has found it.”⁵³ In Manuscript F, he devotes three pages to detailed descriptions of this process for the fossils of mollusks (“animals that have their bones on the outside”), fish (“animals that have their bones within the skin”), and for the impressions of leaves.⁵⁴ The description of the fossilization of fish, in particular, is a dazzling display of Leonardo’s vivid and accurate imagination:

As the rivers abated over time, these creatures, embedded and shut in with the mud, with their flesh and organs worn away and only the bones remaining, but having lost their natural arrangement, fell to the bottom of the mold formed by their impression. And as the mud rose above the level of the river, it dried up and formed first a sticky paste, and then changed into stone, completely sealing up what it contained, filling every crevice.

And having found the hollow animal’s imprint, it penetrated gradually through the tiny fissures in the earth from where the air escaped sideways. It could not escape upward as a result of the sediment which had fallen into the cavity, nor downward because the sediment that had already fallen had blocked up the porosity. There remained only the side openings, from which the air, condensed and under pressure from the action of the descending sediment, escaped with the same slowness as the sediment settled there; and in drying, this paste became stone, devoid of graininess, and it preserved the shapes of the creatures that had left their imprint, and enclosed their bones within it.⁵⁵

The Formation of Mountains

With his meticulous observations and compelling reasoning, Leonardo demonstrated that the marine fossils found in mountain rocks had been formed in the fluvial and oceanic environments where these creatures lived in the distant past. His brilliant arguments invalidated the theories that were current in his time, but they did not solve the fossil enigma, as he well recognized. He still had to show how those layers of marine sediments ended up in the high mountains. In other words, he needed a theory of how mountains were formed during the extremely long periods of geological time. Leonardo did not hesitate to take on this formidable challenge.

The origin of mountains and, more generally, the formation of the Earth were topics that had been discussed by philosophers since antiquity.⁵⁶ Leonardo was familiar with many of the classical texts, including Ptolemy’s celebrated Cosmographia and Pliny’s encyclopedic Natural History, which contained summaries of the writings of almost five hundred Greek and Roman authors.⁵⁷

In Greek philosophy, there were two schools of thought regarding the formation of mountains. Eratosthenes, a contemporary of Archimedes in the third century B.C., believed that the level of the Mediterranean Sea had been much higher in the past, when both the Strait of Gibraltar and the Bosphorus Strait had been closed, and that at some time the “Pillars of Hercules” (Strait of Gibraltar) had burst open, thus lowering the sea level and making mountains appear. The proofs of the former sea level, according to Eratosthenes, were found in the occurrence of fossils in those mountains. The other school of thought was promoted by the geographer Strabo in the first century B.C. According to him, mountains originated in earthquakes, volcanic eruptions, and other natural catastrophes.

Neither of these two theories was entirely satisfactory, and their relative merits were debated for centuries. The inhabitants of the Mediterranean countries were quite familiar with volcanoes, and volcanic eruptions were featured in many legends, such as that of Atlantis. But volcanic eruptions could not explain the deposits of shells and other fossils, as Strabo himself acknowledged. Eratosthenes, on the other hand, did not explain what kind of catastrophe had opened the Pillars of Hercules.

As far as the more general question of the formation of the Earth was concerned, there were also several opinions. Aristotle taught that the Earth was eternal, and that it continually repaired the degradations caused by erosion with the creation of new mountains. The Stoics, on the other hand, expounded the view that the visible signs of decay were proofs that the Earth would eventually perish. However, they believed that it would then recreate itself, and that in this way successive worlds would arise again and again.

Both of these were purely philosophical views, expounded without any attempts to describe corresponding geological processes. An interesting and much more detailed account was given by the Roman poet and philosopher Lucretius in the first century B.C. He maintained that the Earth was formed out of a chaotic mass of atoms of all kinds. As matter condensed, Lucretius explained, the ground began to sink. However, in some places this process was obstructed by the accumulation of rocks, and thus mountains were formed in those areas.

In the Middle Ages, the knowledge of Greek and Roman antiquity had been largely forgotten in Western Europe, but it was assimilated by Islamic scholars who translated the classic texts into Arabic and added their own commentaries and innovations.⁵⁸ The Arab natural philosophers revived the classical debate between the schools of Eratosthenes and Strabo, which they renamed as the theories of “neptunism” and “plutonism.”

According to neptunism, named after the Roman god of the sea, land is continually worn down by erosion and the eroded earth is carried into the sea, where hills and mountains gradually build up from the deposits. This causes the water level to rise, while the height of the mountains on land slowly decreases. At a certain moment, the sea overflows its basin, invades the land, and exposes mountains that were formerly under water. In this way, plains change into oceans, and oceans into plains and mountains. These changes, the neptunists believed, occur periodically in vast cycles of 36,000 years.

To our modern minds, this theory sounds strange, as it is clearly inconsistent with the basic laws of gravity and the flow of water. Yet the neptunist belief that all mountains were formed by layers of deposits at the bottom of the oceans was favored by naturalists until the nineteenth century.

Opposed to neptunism was the theory of plutonism, named after the Greek god of the underworld, which postulated vertical forces in the Earth’s interior that were capable of lifting or lowering land. Proponents of plutonism in the tenth and eleventh centuries included the great physician Avicenna (Ibn Sina).⁵⁹ Two centuries later, the scholastic philosopher Albertus Magnus, who was the teacher of Thomas Aquinas, proposed a similar theory, according to which mountains were uplifted by vapors released from the interior of the Earth.

In spite of the reputations of these scholars, however, the theory of plutonism (or “vulcanism,” as it also came to be called) was not generally accepted in subsequent centuries, because it required the assumption that there was fire at the center of the Earth. Today we know that the core of the planet does consist of hot molten rock, known as magma.* But during the Middle Ages and the Renaissance the idea of a central fire in the Earth was inconceivable because it contradicted the accepted Aristotelian cosmology.

According to Aristotle, the four elements naturally arranged themselves in concentric spheres with earth at the center, surrounded successively by the spheres of water, air, and fire (or light).⁶⁰ He explained that the elements were constantly disturbed and pushed into neighboring spheres, but would then naturally return to their proper places. Hence, the medieval philosophers could accept the occasional existence of fire within the Earth, as evidenced by volcanoes, but could not conceive of fire occupying a permanent place at the Earth’s center. Aristotle’s authority was so powerful that the idea of subterranean fire, or heat, was seriously considered only in the eighteenth century when Hutton used it to explain the igneous origin of granite.

Christian theologians during the Middle Ages had no unanimity about cosmology. In the Old Testament, the Earth is described as a flat disk floating on the ocean, with the firmament held up by pillars above it. However, the modern notion that everybody in medieval times believed that the Earth was flat is no more than a popular cliché. In actual fact, most scholars, from antiquity to the Renaissance, knew that the Earth is spherical.⁶¹

A far greater obstacle to the proper understanding of geological processes was the biblical teaching that the world was created a mere 6,000–8,000 years ago. Today we know that even the much longer period of 36,000 years proposed by Arab scholars in their theory of neptunism is far too short to account for significant geological changes.†

The first to expand the notion of geological cycles was the French scholastic philosopher Jean Buridan in the fourteenth century. According to Buridan, the Earth consists of two hemispheres, one entirely aqueous and the other terrestrial, which periodically interpenetrate each other, so that oceans turn into land and vice versa in enormously long cycles. Leonardo learned about Buridan’s theory through the writings of Albert of Saxony, a disciple of Buridan, and he used the notion of the two hemispheres as an important element of his own theory of the formation of mountains.⁶² But whereas Buridan and Albert believed that the Earth’s asymmetry was created and regulated by God, Leonardo attempted to explain the complex dynamics of the Earth’s shifting balance and the ensuing uplift of mountains entirely in terms of natural causes.

Leonardo’s Tectonic Theory

Leonardo was well acquainted with the principal texts on the formation of the Earth by Islamic and Christian medieval scholars, and he used some of their key ideas to formulate his own theory. It is an elaborate blend of Aristotelian and medieval ideas combined with his own observations, and, like all of his geology, it includes some ideas that sound quite modern. In view of the fact that a proper understanding of the uplift of mountains in terms of plate tectonics was achieved only in the twentieth century, Leonardo’s tectonic theory is indeed exceptional.

Leonardo begins his arguments by noting that, if the world were composed entirely of water, it would form a perfect sphere, like a drop of dew: “The surface of the sphere of water does not move from its perimeter around the center of the world which it surrounds at an equal distance.”⁶³ Then he introduces Buridan’s idea of the two interpenetrating bodies, one aqueous and the other terrestrial. However, in contrast to the scholastic philosopher, Leonardo reasons like a modern scientist, first citing an experiment and then using empirical evidence to construct a simple geometric model of the Earth:

A drop of dew, perfectly round, affords us an opportunity to consider … how the watery sphere contains within itself the body of earth without the destruction of the sphericity of its surface. If you take a cube of lead the size of a grain of millet, and by means of a very fine thread attached to it you submerge it in this drop, you will see that the drop will not lose any of its original roundness, although it has been increased by an amount equal to the size of the cube which has been enclosed within it.⁶⁴

Instead of a small cube, Leonardo then reasons, we could also imagine a terrestrial pyramid, or tetrahedron, immersed within the sphere of water, with its corners protruding into the air. This is his model of the Earth, presented in a simple sketch in the Codex Leicester (fig. 2-7), a mass of land partially surrounded by water.

FIG. 2-7. Geometric model of the Earth. Codex Leicester, folio 35v (detail).

What is remarkable here is not only Leonardo’s ingenious use of a simple theoretical model—a technique that would become an integral part of the scientific method in subsequent centuries—but also his utterly dynamic image of the Earth as a mass of land floating in water. The conception is not unlike that in modern plate tectonics, except that in the modern theory the land masses are conceived as drifting on a plastic layer of partially molten rocks (see p. 93).

Having illustrated the dynamic relationship between land and water with his geometric model, Leonardo then proceeds to discuss the actual shape of the Earth.⁶⁵ He explains that the Earth has a geometric center, which he calls the “center of the world.” If the mass of land were homogeneous and equally distributed, it would form a perfect sphere around that geometric center, and so would the sphere of water surrounding the element earth.

In reality, however, the mass of land is far from homogeneous. The interior of the world, in Leonardo’s view, is a complex conglomeration of solid earth and rocks, water running through various conduits, and large caverns hollowed out by erosion. Because of this unequal distribution of land and water, one hemisphere of the Earth will always be heavier than the other, and hence the Earth’s “center of gravity” will not coincide with the “center of the world.”* Dividing the sphere horizontally with the heavier hemisphere at the bottom, Leonardo pictures the center of gravity as lying below the geometric center, within the heavier hemisphere.

However, the relative positions of the two centers of the Earth are not static. As in the model of the terrestrial pyramid within the sphere of water, the land masses of the Earth can slide deeper into the sphere of water or emerge farther from it. To explain the forces causing such movement, Leonardo introduces an Aristotelian argument. As a living organism, he reasons, the Earth will naturally move toward a state of balance, and hence will strive to bring its center of gravity closer to the geometric center.

As Gould has pointed out, this movement may be likened to that of a seesaw.⁶⁶ Just as two people of unequal weight can balance a seesaw if the heavier person moves inward and the lighter person moves outward, so the solid masses of the heavier hemisphere will gradually sink toward the center of the world, while the rocks of the lighter hemisphere will rise out of the water. And this is how, according to Leonardo, layers of sedimentary rock emerge from the sea to form mountains:

And so the lightened side of the Earth is continually raised, and the antipodes draw nearer to the center of the Earth, and the ancient beds of the sea become chains of mountains.⁶⁷

In Leonardo’s view, water has continually shaped the forms of the Earth, and thus erosion is the major cause for the unequal distribution of mass in the two hemispheres. He saw the uprising of mountains and their erosion as different stages of the same cycle of transformation that took place over enormous periods of geological time. In the Codex Atlanticus, he noted how the uplift of the mountains is always followed by their erosion:

I maintain that … the mountains, the bones of the Earth, with their wide bases penetrated the air and rose up into it, covered over and clad with much high-lying soil. Subsequently the frequent rains and the swelling of the rivers by repeated washing stripped bare part of the lofty summits of these mountains, so that the rock finds itself exposed to the air and the earth has departed from these places.⁶⁸

And in Manuscript F, Leonardo explains how the erosion of mountains, in turn, causes them to rise up farther:

The Earth is always growing lighter in some part, and the part that becomes lighter pushes upwards, and submerges as much of the opposite part as is necessary for it to join its … center of gravity to the center of the world.⁶⁹

Today, the enormous cycle of erosion, sedimentation, uplift, and renewed erosion is known to geologists as the rock cycle.* The idea was central to Leonardo’s conception of the Earth as dynamic and continually changing, and it is one of several reasons why his geology sounds so modern.

In addition to the gradual erosion of the mountains and their concurrent gradual uprising, Leonardo’s tectonic theory also includes catastrophic events that take place in the interior of the Earth. Since antiquity, it was believed that there were numerous water veins inside the Earth (see p. 26), and Leonardo imagined that this water, over time, had carved out huge subterranean caves. As their erosion continued, the caves would eventually become unstable. Parts of the Earth’s crust would collapse into them, and the movement of these enormous masses of rock toward the center of the world would lighten the upper hemisphere and thus contribute to the uplift of its mountains. In the Codex Leicester, Leonardo provides a clear description of this imaginary tectonic process:

The very large space of the Earth that was filled with water, that is the immense cavern, must have had a considerable portion of its vault fall toward the center of the world, finding itself detached by the subterranean water courses that continually wear away the place through which they pass…. Now this great mass could fall … and it made the Earth lighter at the point where it broke off, and that part of the Earth immediately moved away from the center of the world and rose to the height where one sees the layered rocks, produced by the orderly action of the waters, at the summits of the high mountains.⁷⁰

Il grand’amore nasce dalla gran
cognitione della cosa che si ama. (Trattato della pittura, chapter 80)

Great love is born of great knowledge
of the thing that is loved.

The theme of catastrophic collapse in subterranean caves was taken up again in subsequent centuries and was discussed under the name of “catastrophism” until the emergence of plate tectonics in the twentieth century.⁷¹ The modern theory of plate tectonics provides a unifying conceptual framework for understanding the composition, structure, and internal processes of the Earth.⁷² Its key idea is that the lithosphere, the hard outer layer of the Earth, is composed of giant plates that move over a plastic layer of partially molten rocks, known to geologists as the asthenosphere. At their boundaries, these plates may move apart, slide sideways past each other, sink beneath one another, or collide head on. These extremely slow interactions of massive plates generate the forces that create, respectively, mid-oceanic ridges, earthquakes, volcanic island arcs, and mountain ranges. The force that drives the movements of the plates has to do with circular currents of hot magma in the Earth’s mantle underneath the lithosphere. The details of these thermodynamic processes are still not fully understood.

In Leonardo’s tectonic theory, the large masses of land float in water rather than in hot magma (the element of fire). Nevertheless, the general features of his explanation of the origin of mountains have much in common with the modern theory. In both cases, the rising up of mountains is a consequence of catastrophic events that involve enormous masses of rock shifting inside the Earth. And even Leonardo’s idea of land masses slowly rising out of the water when they become lighter, in order to keep land and water in balance, has an equivalent in modern geology. According to the so-called isostatic theory, parts of the upper crust that float on the layer of molten rocks will rise up vertically when they become lighter, just as they do in Leonardo’s theory.⁷³ Modern geologists have confirmed, for example, that northern Europe is still rising because of such isostatic adjustment, triggered by the melting of the ice cap that covered the region a million years ago. Once again, it is astonishing to realize the prescient nature of Leonardo’s scientific conceptions, especially when they concern processes of enormous scale that take place over immensely long periods of time.

The Distant Geological Past

During the years 1502–4, Leonardo traveled widely in Tuscany and Ro-magna in connection with various projects of military and hydraulic engineering.⁷⁴ While he drew beautiful and ingenious maps of the regions he visited, he also undertook extensive studies of their geological formations, observing and recording them with remarkable accuracy. “Everyone, and especially the field geologist,” notes Ellenberger, “will appreciate the inestimable value and be amazed at the freshness and perspicacity of these observations and their interpretation.”⁷⁵

Many of these observations took place in the Arno basin, both upstream and downstream from Florence. In one entry in the Codex Leicester, for example, Leonardo noted:

Near Monte Lupo, [the Arno] left gravel deposits, and these are still to be seen welded together, forming of various kinds of stones from different localities and of varying color and hardness one solidified mass. And a little further on, where the river turns toward Castel Fiorentino, the hardening of the sand has formed tufa stone. Below this, it deposited the mud in which the shells lived; and this has risen in layers acccordingly, as the floods of the turbid Arno were poured into this sea … as is shown in the cutting of the Colle Gonzoli, laid open by the Arno, which wears away its base.⁷⁶

As he had depicted the differences between igneous rock and sandstone in the Virgin of the Rocks twenty years earlier, Leonardo now clearly perceived the differences between mountains formed from layers of hard rock (known to modern geologists as turbidite) and layers of clay, full of fossils, at the foot of these mountains. A sketch in the Codex Leicester (fig. 2-8) illustrates the difference between these two types of sediment, which date from different geological periods: deposits of clay at the foot of a mountain of horizontally layered and much older turbidite.

Leonardo used his observations in the Arno valley to speculate about the basin’s geological history. He had known since his childhood that downstream of Florence, near Vinci, the plain of the Arno is intersected by a transverse chain of hills, Monte Albano and Gonfolina, and he speculated that in the distant past, the basin of Florence and that of Arezzo, further upstream, were the sites of two giant lakes at different levels:

In the great valley of the Arno above Gonfolina, a rock was united since antiquity with Monte Albano in the form of a very high bank. This kept the river dammed up in such a way that, before it could empty itself into the sea … it formed two large lakes, the first of which is where we now see the city of Florence flourish together with Prato and Pistoia…. In the upper part of the Val d’Arno, as far as Arezzo, a second lake was formed, and this emptied its waters into the aforementioned lake.⁷⁷

FIG. 2-8. Sketch of clay deposits at the foot of a mountain of horizontally layered sandstone. Codex Leicester, folio 36r (detail).

In subsequent years, the image of interconnected mountain lakes flowing into one another became an important visual metaphor in Leonardo’s paintings, illustrating the continual flows and transformations that have shaped the surface of the Earth over immense periods of geological time. These lakes are clearly visible in the backgrounds of his mature masterpieces, the Mona Lisa and the Madonna and Child with Saint Anne (see plates 7 and 11). In both paintings, Leonardo portrays his vision of the distant geological past—the forms of the Earth in ceaseless movement and transformation. As the mountains rose up from the primeval ocean, pockets of inland seas and lakes were created that would eventually find their way back to the ocean as their waters cut narrow gorges into mountains and hills. Gradually, these waters would carve out valleys and deposit in them masses of gravel and sand that eventually would become fertile soil. In other words, the mythical rock formations in these two master paintings represent Leonardo’s meditations on the birth of the living Earth.