Much of historical geology is based on a relationship called the LAW OF
SUPERPOSITION. This simply means that when some rock formation was
placed on top of some other formation by natural processes
(sedimentation or volcanic eruption, for example), the layer on top must
be younger than the one on the bottom. Such a conclusion may now seem
obvious, but the concept was not even expressed until the very end of
the eighteenth century and was still a matter of scientific controversy
when Abraham Lincoln was a boy. It was the law of superposition,
however, that led the early geologists to establish the first geologic
time scales and to realize the enormous extent of geologic time.
[Illustration: _Diagram illustrating the law of superposition. Each rock
bed is younger than the strata under it._]
In essence the system of establishing age by this concept is this:
Somewhere a large and easily recognized layer of sedimentary rock was
known. It had a characteristic color, texture, gross composition, and
overall appearance. Let us call it _bed M_. This bed could be traced
across the countryside until a place was reached where it could be seen
that _bed M_ rested on another, different layer of rock, which we might
call _bed L_. _Bed L_ could also be traced some distance and ultimately
could be observed resting on a still different stratum, which we shall
call _bed K_. Some of these beds had fossils in them, and it was
eventually realized that rocks with the same kinds of fossils are of the
same age, even though they may differ in other respects—in color or
composition, for instance.
If _bed M_ was followed in another direction, perhaps a point was
reached where it dipped down a little, and here, was found still another
layer—call it _bed N_—on top of _M_. Obviously, the sequence of beds
from oldest to youngest was _K-L-M-N_. Their relative ages were now
established. Over the years, hundreds of geologists described various
rock layers and identified the fossils in them. By the middle of the
nineteenth century, this “layer-cake” structure of sedimentary rocks was
well-known in western Europe. (One may say, parenthetically, that
America was geologically a vast unknown at that time. Today we know much
more about the geology of Antarctica than anyone a hundred years ago
knew about the geology of the United States.)
A system of nomenclature for the “layer-cake” was developed and refined.
Gradually this nomenclature was accepted internationally. Long-range
correlations between beds of the same age, distant halfway around the
globe from each other, were made possible as the science of PALEONTOLOGY
developed. The relative age of almost any rock containing even poorly
preserved fossils could be determined anywhere in the world with
precision. That is, the age of rock layers in relation to one another
was known. But the real age—the absolute age—remained unknown until
knowledge of radioactivity provided the necessary clocks.
[Illustration: _A geologist carefully maps the layers of soil in which
ancient flint tools have been found. Exact correlation of the
tool-bearing strata with material datable by carbon-14 analysis must be
accomplished before the age of the tools can be determined._]
Even this process wasn’t entirely without problems. The difficulty lay
in the fact that ordinary sedimentary rocks (shale, sandstone, and
limestone) cannot be dated by the usual nuclear methods because they
present no suitable closed systems. Only some volcanic sediments can be
reliably dated by the mica, feldspar, and zircon they contain; but these
ancient ashfalls are rare, usually are only a few inches thick, and are
not easy to identify at the surface because they weather quickly to
clay. Only about a dozen volcanic beds have been accurately dated in
North America. The time points they established are mainstays of the
present geologic time scale, but they have had to be supplemented by
indirect information.
[Illustration: _Steeply dipping sandstone strata from the Cretaceous
Period near Gallup, New Mexico. This photo, taken in 1901 (note the
horse-drawn wagon), was made on an early government survey of Western
lands._]
[Illustration: _Fossils of bird tracks in sandstone in Death Valley,
California._]
The most important of these indirect time points are furnished by what
geologists call BRACKETED INTRUSIVES. It often happens in geologic
history that a mass of rock becomes molten at a great depth and forces
its way up through several layers of sedimentary rocks. The sedimentary
layers are usually bent and twisted (folded) by the upthrust, and where
the cooler rock comes in contact with the molten mass, cooler material
is burned (recrystallized). Geologists call that process CONTACT
METAMORPHISM because the sedimentary rock forms are changed, or
metamorphosed, to have another form or composition.
Contact-metamorphosed rocks, in spite of the damage they have suffered,
may contain recognizable and accurately datable fossils. Thereby the
metamorphic rock establishes a lower limit for the age of the intrusion:
It must be younger than the fossils in the youngest of the metamorphic
rocks it touched.
Now on top of the intrusive rock, we may find another sedimentary rock,
deposited on top of the intrusive after it cooled and was exposed by the
erosion of overlying materials, perhaps millions of years later. This
new sediment may also contain fossils and thus furnish an upper limit
for the age of the intrusion. The measured age of the intrusive rock
thus can be used to set upper and lower limits on the absolute ages of
the two sediments. If we are lucky, the two sediments will bracket a
relatively short interval, making our measurement quite precise.
[Illustration: _Idealized sketch of a bracketed intrusive. The igneous
(molten) rock must be younger than the sedimentary rock (A) it intrudes,
and older than the rock (B) that overlies it. The relative age of the
sedimentary beds is known from their fossils._]
[Illustration: _Facsimile reprint of the famous time scale proposed by
Arthur Holmes in 1959._]
Time-scale in millions of years
PERIODS Since beginning of period Duration of period
PLEISTOCENE _ca_ 1
.... _ca_ 1 ....
PLIOCENE 10
.... 11 ....
MIOCENE 14
.... 25 ....
OLIGOCENE 15
.... 40 ....
EOCENE 20
.... 60 ....
PALEOCENE 10
.... 70 ± 2 ....
Upper }
CRETACEOUS 65
Lower }
.... 135 ± 5 ....
Upper }
JURASSIC 45
Mid. & Lower }
.... 180± 5 ....
TRIASSIC 45
.... 225 ± 5 ....
PERMIAN 45
.... 270 ± 5 ....
Upper }
CARBONIFEROUS 80
Lower }
.... 350 ± 10 ....
Upper }
DEVONIAN 50
Lower }
.... 400 ± 10 ....
SILURIAN 40
.... 440 ± 10 ....
ORDOVICIAN 60
.... 500 ± 15 ....
CAMBRIAN 100
.... 600 ± 20 ....
Using measurements made in many laboratories, and interpolating between
them by using the relative thicknesses of sediments, the great British
geologist, Arthur Holmes, established the time scale that is in general
use today. His original scale is shown on the preceding page. Small
changes can be expected to be made in this scale from time to time, but
major alterations are not likely, except perhaps in the Cambrian Epoch
where the present data are unreliable because they are not complete. (A
scale showing the epochs or periods as often given now is on page 4.)