IN CONTEXT

BRANCH

Geology

BEFORE

Late 15th century Leonardo da Vinci writes about his observations of the erosional and depositional action of wind and water on landscapes and surface materials.

AFTER

1780s James Hutton refers Steno’s principles to a continuing and cyclical geological process stretching back in time.

1810s Georges Cuvier and Alexandre Brongniart in France and William Smith in Britain apply Steno’s principles of stratigraphy to geological mapping.

1878 The first International Geological Congress in Paris sets out procedures for the production of a standard stratigraphic scale.

The sedimentary strata of rocks that make up much of Earth’s surface also form the basis for Earth’s geological history, which is normally depicted as a column of layers with the oldest strata at the bottom and the youngest at the top. The process of deposition of rock by water and gravity had been known for centuries, but Danish bishop and scientist Niels Stensius, also known as Nicolas Steno, was the first to describe the principles that underlie the process. His conclusions, published in 1669, were drawn from his observations of geological strata in Tuscany, Italy.

Steno’s Law of Superposition states that any single sedimentary deposit, or stratum, is younger than the sequence of strata upon which it rests, and older than the strata that rest upon it. Steno’s principles of original horizontality and lateral continuity state that strata are deposited as horizontal and continuous layers, and if they are found tilted, folded, or broken, they must have experienced such disturbance after their deposition. Finally, his principle of cross-cutting relationships states that “if a body or discontinuity cuts across a stratum, it must have formed after that stratum”.

  Steno’s insights allowed the later mapping of geological strata by the likes of William Smith in Britain and Georges Cuvier and Alexandre Brongniart in France. They also allowed the subdivision of strata into time-related units, which could be correlated with each other across the world.

Rock strata, as Steno realized, all start life as horizontal layers, which are subsequently deformed and twisted over time by huge forces acting on them.

See also: James Hutton • William Smith