Steno, who was born on January 11, 1638, wrote a paper on fossiliferous rocks in which he outlined what are now known as Steno’s principles of stratigraphy. He recognized that layering was the most conspicuous property of sedimentary rocks, and his three principles help us to decipher the history of a succession of sedimentary rocks:
- Principle of superposition — Unless deformed by folding or faulting, a sequence of sedimentary layers will progress in age from oldest to youngest as one goes from bottom to top.
- Principle of original horizontality — Because particles settle from fluids, such as water, most sedimentary strata are formed as horizontal layers. When one sees steeply tilted layers in mountains, for example, we can know that they were once horizontal and have been deformed through mountain-building processes, such as folding or faulting.
- Principle of lateral continuity — Strata were originally deposited as continuous layers, though those layers may now be broken by subsequent erosion. A good place to observe this is the Grand Canyon, where the layers in the North Rim correlate to the same layers in the South Rim.
Steno also made a significant contribution to the development of the science of mineralogy by observing that for a given mineral, such as quartz, the angles between different crystal faces are constant from one specimen to another. This is known as Steno’s law of constant angles.
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