Anniversaries for the two founding fathers of geology occurring in the same year prompted a comparative evaluation of how the two contributed to establishing the basic principles of the discipline. To do so, passages from their publications, codices and manuscripts have been quoted directly. The Stenonian principles (‘original horizontality’, ‘original continuity’, and ‘superposition of individual strata’) are present in Leonardo’s notebooks amazingly formulated, using similar wording when studying the same area more than 150 years earlier. Also, Stenonian priority in naming and explaining geological concepts and processes (e.g., faulting, folding, angular unconformity, relative chronology) are mirrored in Leonardo’s writings and pictorial works. While Steno enjoys priority in stepwise restoration of the geological history of a given region, Leonardo was the first to construct a 3D geological profile representation and geomorphologic maps. Lastly, the paper focuses on diverging stances of the two savants about the Noachian Deluge and the age of the Earth. Already 500 years ago, Leonardo had solved the question of marine fossil remains of organic origin found in the mountains implying the possibility of deep geologic time in a statement of ‘eternalism’. 350 years ago, Steno solved the same question in a different way in which he retained a basic role for the Deluge and assumed a short age for the Earth by focusing mainly on short-lived sedimentary and geomorphologic processes.
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