1669

Foundations of Geology

Nicolas Steno (1638–1686)

To a geologist, layered rocks are like the pages of a book, just waiting to be read. But many of the basic principles underlying the scientific study of layered rocks—the field of study known as stratigraphy—had to be painstakingly developed and advocated by early practitioners of geology. Among the first to formalize what have essentially become the bedrock principles underlying all of modern geology was the Danish scientist (and, later, bishop) Nicolas Steno.

Steno was trained in anatomy and medical sciences, and was a consummate observer of features and structures in biological specimens as well as the natural world. After dissecting the head of a shark, he noted striking similarities between its teeth and certain shark-tooth–shaped objects embedded within rocky geologic formations. He soon became a leading advocate of the idea that these and other bone-like structures embedded in rocks were the fossilized remains of once-living organisms.

Steno’s observations of geologic formations took him further, and in 1669 he developed and published a book called Dissertationis Prodromus that laid out what are still the fundamental principles of stratigraphy today: the Law of Superposition (in layered rocks, younger strata are above older strata), the Principle of Original Horizontality (strata in layered rocks were originally formed horizontally), the Principle of Lateral Continuity (layers are continuous across large distances), and the Principle of Cross-Cutting Relationships (layers that cut across or intrude into other layers must be younger). These principles are still taught in introductory geology classes today.

Steno’s stratigraphic principles might seem relatively obvious to us (or to beginning geology students) today, but they were both innovative and controversial in his time. The concept of coherent solid objects like fossils occurring deep within other coherent solid objects such as layered rocks was difficult to reconcile with the classical, Aristotelian view of these features merely being an inherent characteristic of the Earth. Steno’s principles of stratigraphy also hinted at the potential for truly ancient ages of rock formations (indeed, of the entire Earth) and enormous spans of time across geologic layers. However, neither the technology nor the social and religious climate would exist to realize that potential for several more centuries.

Main image: Spectacular folded, layered rocks in the Hajar Mountains, Oman. Inset: Cover of Dissertationis Prodromus (1669) by Nicolas Steno.