is a branch of geology that studies rock strata with
an emphasis on distribution, deposition, age and evidence
of past life.
Nicolas Steno, William Smith, Georges Cuvier, Alexandre
Brongniart, and James Hutton developed the basic rules
for the science
of stratigraphy. Relative dating uses the principles
or laws of stratigraphy to order sequences of rock strata.
Relative dating not only
determines which layers are older or younger, but also
gives insight into the paleoenvironments that formed the
sequence of rock.
A chance encounter between determined
fishermen and a great white shark off the Tuscan coast
in 1666 sparked a chain
of events that
would help change humans views of fossils and Earth’s
geologic past (Cutler 2003, pp. 5-8). Nicolas Steno (1638-1686)
the head of this shark and realized fossil tongue stones
believed to be petrified snake or dragon tongues were actually
shark teeth (Prothero 1998, p. 3). One problem still existed,
fossils become embedded in solid rock? Steno recognized that
organisms that became buried in sediment, which later turned
into rock. The realization that sediments turn into rock
to the view that all rocks on Earth formed in a single creation
Once Steno recognized that the fossils he was contemplating
(sharks teeth and sea shells) were formed in the sediments
he was able to work out the basic rules of stratigraphy.
the laws of superposition, original horizontality, original
continuity and inclusions in his publication entitled De solido intra solidum
naturaliter contento dissertationis prodromus (Prothero 1998, p.
law of superposition is the foundation of Steno’s work
on stratigraphy. The principle
states that in an undisturbed sequence
of strata or lava flows; each layer is older
than the one above and younger than the one below. The
law of original horizontality states
that sedimentary strata and lava flows are deposited in horizontal
sheets. If these layers are not horizontal, subsequent
movements have occurred. The
law of lateral continuity states
that strata and lava flows extend laterally in all directions
out at the edge of their deposition. Inclusions are rock
fragments or fossils contained in another rock
principle of inclusions states
that any inclusion is older than the rock that contains it.
Steno's idea that fossils are older than the rock in which
they are found hints at this principle, but Hutton is most
often given credit for this principle. Steno
developed these principles in the context water deposited
sediment. It is not clear he was aware of igneous rock formed
from lava flows.
The principle of faunal succession states
that fossil organisms succeed one another in a definite, irreversible,
and determinable order.
This law was independently discovered by William Smith (1769-1839),
a British engineer, while working on excavations for canals in England
(Winchester, 2002 p. 131) and by Georges Cuvier (1769-1832), a French
anatomist, and Alexandre Brongniart (1770-1847), a French naturalist
their work on the deposits of the Paris Basin. Brongniart was the
first to use fossils to date rock strata.
James Hutton (1726-1797) is often considered
the father of geology. Hutton developed the theory
of uniformatarianism, which states that
events are caused by natural processes, many of which are operating
in our own time. Put another way, the natural laws that we know
about in the present have been constant over the geologic past.
of geologic time or deep time was a logical consequence of
this theory. In
Playfair came to
see Hutton’s Unconformity in Inchbonny. The unconformity
consists of many vertical tilted layers of grey shale overlaid
by many layers
of horizontal red sandstone. Playfair later commented that, "the
mind seemed to grow giddy by looking so far into the abyss of
(1998) points out that Hutton removed humans from a specious
place in time just as Copernicus had removed humans from
a specious position
in the universe (p. 74).
Hutton gives us three more laws to consider when seeking
relative dates for rock layers, one of which, the law of
inclusions was described earlier. The
law of cross-cutting states
any feature that cuts across a rock or sediment must be younger
than the rock or sediment
through which it cuts. Examples include fractures, faults,
and igneous intrusions. Igneous intrusions are sometimes
referred to as a seperate principle, the principle
of intrusive relationships. Unconformities represent
gaps in geologic time when layers were not deposited
or when erosion
includes three types of unconformities. A disconformity is
an unconformity between
parallel layers. An angular
unconformity exists when younger
more parallel strata overlie tilted strata. A nonconformity is
formed when sedimentary layers
are deposited on igneous or metamorphic rock. Hutton’s
theory of uniformatarianism and the principles of stratigraphy
would be fully developed and made popular by another Scottish
geologist Charles Lyell (1797-1875) with his classic three
volume work, first published from 1830 to 1833, entitled Principles
of Geology: being an attempt to explain the former changes
of the Earth’s surface by reference to causes now in
operation (Levin, 1999, p. 9).
The science of stratigraphy changed humans’ view of
the world from one, which was static to one that was dynamic
Not only did the rock layers indicate changing environments
they also revealed that different life forms have existed
Back to Dating
Cutler, A. (2003). The
Seashell on the Mountaintop. New York:
H.L. (1999). The Earth Through Time [6th Ed.].
New York: Harcourt Brace College Publishers.
J. (1998). Annals of the Former World. New York:
Farrar, Straus, & Giroux
Prothero, D. R. (1998). Bringing
Fossils to Live: An Introduction to Paleobiology. New York: McGraw-Hill.
Winchester, S. (2002).
The Map that Changed the World: William Smith and the
Birth of Modern Geology. New York: Perennial.