The Notebooks of Leonardo Da Vinci Complete


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followed the rest of its bank as far as where Serravalle now stands.  
From the Val d'Arno upwards, as far as Arezzo, another lake was  
>
formed, which discharged its waters into the former lake. It was  
closed at about the spot where now we see Girone, and occupied the  
whole of that valley above for a distance of 40 miles in length.  
This valley received on its bottom all the soil brought down by the  
turbid waters. And this is still to be seen at the foot of Prato  
Magno; it there lies very high where the rivers have not worn it  
away. Across this land are to be seen the deep cuts of the rivers  
that have passed there, falling from the great mountain of Prato  
Magno; in these cuts there are no vestiges of any shells or of  
marine soil. This lake was joined with that of Perugia [Footnote:  
See PI. CXIII.]  
A great quantity of shells are to be seen where the rivers flow into  
the sea, because on such shores the waters are not so salt owing to  
the admixture of the fresh water, which is poured into it. Evidence  
of this is to be seen where, of old, the Appenines poured their  
rivers into the Adriatic sea; for there in most places great  
quantities of shells are to be found, among the mountains, together  
with bluish marine clay; and all the rocks which are torn off in  
such places are full of shells. The same may be observed to have  
been done by the Arno when it fell from the rock of Gonfolina into  
the sea, which was not so very far below; for at that time it was  
higher than the top of San Miniato al Tedesco, since at the highest  
summit of this the shores may be seen full of shells and oysters  
809  


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