The Science Book

55

T

he 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 crosscutting

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. ■

SCIENTIFIC REVOLUTION

LAYERS OF ROCK

FORM ON TOP OF

ONE ANOTHER

NICOLAS STENO (1638–1686)

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.

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 96–101 ■ William Smith 115