The Medieval & Modern Geology of Leonardo da Vinci
by Gretchen Noyes-Hull
Imagine writing down your cleverest ideas and most thoughtful musings in a diary or journal. Figuring that sooner or later you would come back to them, you close the covers and slide the book to the back of your bookshelf, behind all the other volumes. Imagine that years later, when clearing the shelves, you rediscover your journal. What would you find? And how would you feel?
Perhaps you would discover that during the time that had passed, other people had come up with some of your ideas. You might also find that your own thoughts would have changed somewhat, for they were products of your age and understanding at the earlier time. No doubt you would be secretly proud that others had come up with some very similar ideas well after you had, and you also would be interested in comparing your earlier thoughts with what you know now.
Leonardo da Vinci also would be pleased with himself if he were with us today! He would not be surprised that the concepts and observations he had so diligently recorded were still highly regarded! He was well aware that his ideas on geology, in particular, were radical and even dangerous for his time. But he would be intrigued by the differences between his thoughts on the subject and ours; his ideas were rooted in another age, 500 years ago.
In fact, for more than 300 of those years the notebooks and journals into which Leonardo had poured his thoughts were scattered and lost on a great many bookshelves. While his notes were sidetracked from the progression of science, many of the same geological principles that he had recorded were independently “rediscovered” by his unknowing successors. And during that time, a whole new method of scientific thinking had become generally accepted.
“Man is the model of the world,” Leonardo wrote. Immersed in a 2,000-year-old tradition that dated from the days of the Greek philosopher Aristotle, he believed that both man and earth were composed of four elements: earth, water, air, and fire. On earth, the elements have a tendency to settle in four concentric layers, from heaviest to lightest. But complete stability is never reached: Like the human body after which it is modeled, the earth is always changing. Water flowing down from mountain peaks takes rocks to the sea.
Leonardo set out to understand the circulation of water and the cycling of earth. He believed that “the same cause that keeps the blood at the top of a man's head keeps water at the summit of mountains.” But what about the mountains?
The circulation of water on earth was a problem. Leonardo could only surmise that water must circulate from the sea to mountaintops in underground “veins.” But he had actual evidence of the mountain-making: the occurrence of ocean fossils high on hills and peaks far from the sea. He rightfully postulated that these shells had been deposited in bottom sediments of shallow seas before being elevated in the form of mountains.
Leonardo made extensive and very thorough observations of fossil shells in the Italian countryside in order to disprove the accepted theories for their existence and to support his own ideas about the circulation of water and earth. He meticulously recorded his findings in the notebook known today as the Leicester Codex.
It is very apparent that Leonardo had no patience for those who supported other explanations for the fossils. The interweaving of the biblical flood story with an ancient Greek belief that lifelike shapes are created in stone according to the stars was particularly offensive! He writes of the “stupidity and ignorance of those who imagine that these creatures were carried by the Deluge to such places distant from the sea,” and criticizes the other “set of ignoramuses for thinking that nature or the heavens have created them or these places through celestial influences.” Additionally, he sputters: “Such an opinion cannot exist in brains possessed of any extensive powers of reasoning…”!
There was nothing wrong with Leonardo's own reasoning powers. Step by step, he provided data to take apart views that conflicted with his own. How could shell shapes grow to different sizes within rocks without cracking them? And if there were a worldwide flood, where would the water drain when it was all over, he asked. The evidence did not support the concept of a universal flood. There was no way that live corals and mollusks could travel as far as 400 kilometers inland in 40 days and 40 nights, nor up mountainsides. He noted that many of the bivalve shells appeared as they had in life, with the two shells still joined.
If, he said, “the shells had been carried by the muddy deluge they would have been mixed up, and separated from each other amidst the mud, and not in regular steps and layers—as we see them now in our time.”
The different strata of now high-and-dry sedimentary rock could only have been formed from the deposits of not one, but multiple floods, occurring over long periods of time. Leonardo concluded that the fossils had been raised up.
The realization that the geological processes that take place today are the same as those that occurred in the long distant past is an extremely modern view. Scientists today also study living organisms to learn about the conditions under which their ancestors were deposited and fossilized. Like Leonardo, for instance, we assume that when the two shells of a bivalve mollusk fossil remain connected, the animal probably died in place. When fossil shells are all jumbled and broken, we imagine that they were once swept together by waves on an ancient beach.
It would take more than 200 years for this type of reasoning to become an accepted principle of geology. With the manuscripts dismantled and scattered, future scientists had to rediscover Leonardo's observations conserning the immensity of geological time, sedimentary layering, and the significance of fossils. Until almost 1800, most people believed that the earth was unchanging.
Leonardo had understood that mountains lifted marine sediments in their formation. He had also believed that mountain-making was part of a cycle that included their erosion. Today we know that the earth's surface has been recycled many times. The collision and crumpling of tectonic plates giving rise to mountains is part of that cycle. We understand why seashells can be found high on the summits of the Alps, the Andes, and the Himalayas. They are all remnants of organisms buried in the sediments of shallow seas. Leonardo would be pleased.
- postulate: Assume.
- strata: Horizontal layers of sediment.
- surmise: Guess.
- tectonic plate: An irregular section of the lithosphere that floats on Earth's mantle.
- From where did Leonardo believe the shells found in the mountains of Italy had come?
- Think about what you learned in Lesson 3 about how different kinds of mountains form. From what kind of mountains do you think the shell fossils came? Why?
- Do you think different kinds of fossils would be found in different types of mountains? Explain your answer with a few sentences.