The Mediterranean basin and the making of valleys
How the sea drained through the Strait of Gibraltar; rivers, valleys and mountain lakes
In two columns of cosmographic geology Leonardo argues that the Mediterranean, once far higher, was made lighter as its waters descended into the Ocean and hollowed the sea floor back toward the Strait of Gibraltar (the 'Gaditanian cut'), while the earth rose and shifted its center of gravity. He compares this to mountains falling in his own day to dam valleys and form lakes such as Garda, Como, Lugano and Maggiore, and calls the Mediterranean the greatest 'river' of the world, running some 10,000 miles from the source of the Nile to the western Ocean. A small map-like figure of the Mediterranean basin marks with the letter a the point where its water fell into the Ocean, and columns of figures accompany the text.
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The lowered sea, the risen earth and mountains that dam valleys
As the diminished Mediterranean waters lost weight, the earth rose and changed its center of gravity, while the outrushing sea hollowed the floor back toward the Gaditanian channel. Leonardo compares this to a mountain of seven miles falling in his own time to shut a valley and make a lake, as with the Alpine lakes of Garda, Como, Lugano and Maggiore.
The Mediterranean as the greatest river of the world
Every valley is made by its river, and the plains between seas and mountains were once covered by salt water. The greatest 'river' is the Mediterranean itself, running from the source of the Nile to the western Ocean, its supreme height in outer Mauretania, a course of 10,000 miles reckoned as 3,000 for the Mediterranean, 3,000 for the visible Nile and 3,000 for the eastward Nile.
Figure of the Mediterranean basin, point a
A schematic basin map accompanies the text, with the letter a marking the place where the water of the Mediterranean fell into the Ocean. The wavy outline sketches the enclosed sea and its outlet toward the Atlantic.