This an area of common misconception. Most sedimentary rock, and
the fossils contained therein cannot be dated directly with
radiometric techniques, but can be dated indirectly. Here's the
Because most sedimentary rock is composed of particles of
pre-existing rock of various types, each particle can be a
Carbon-14 or carbon dating techniques can only be used to date
organic material which is of a maximum age of 70,000 years; this
leaves out the vast majority of fossils found in rock, and only
covering roughly .000016 percent of geologic time.
The dating of fossils was originally done by their placement in
a geologic column of rock strata. Because of the Law of
Superposition, the deeper the stratum, the older it is (in an
undisturbed body of rock or sediment). Fossils which were evident
in a particular stratum, but missing from other strata were noted
as "index" fossils. The presence of these fossils indicated the age
of the stratum relative to other layers. The layers of stratum were
placed in a geologic column, subdivided, and assigned names based
on the presence of different types of fossil organisms. The use of
fossils in this manner is known as biostratigraphy and is the basis
of "relative dating" of rock layers and fossils, a dating technique
that was in effect until the advent of radiometric techniques in
the twentieth century.
Radiometric dating of rock involves the measurement of the
amount of decay in radioactive elements which are present in the
rock. Radioactive elements decay at unique rates, depending on the
isotope. This rate of decay is known as half-lives, it is the time
necessary for ½ of the atoms to decay in a particular element. The
decay follows a geometric scale, in that in the first half-life of
an element, ½ of the atoms decay, yet in the second half-life, ½ of
those remaining decay and so forth. By measuring this decay, and
knowing the half life of an element, scientists can date a sample.
Radiometric dating is particularly useful in dating igneous and
Because of geologic events such as plate tectonics and
volcanism, suitable material for radiometric dating, such as
volcanic tuff, solidified lava, and igneous intrusions have been
found as layers on, in, and cut through layers of sedimentary rock.
Intrusions are always younger than the rock body they penetrate,
meaning that the sedimentary rock in which intrusions are found
will be older than a radiometrically dated sample of the intrusion.
Lava flows and volcanic ash which form layers in rock will be
younger than the rock below and older than the rock above. The
solidified intrusions and lava flows can be dated with radiometric
The radiometric dating of suitable rocks (i.e. igneous or
metamorphic) in proximity to their sedimentary counterparts,
therefore allows the sedimentary rock to be dated as well.