One apparently obvious way to correlate (tie together) these local sequences might be to trace the layers exposed in
our outcrop laterally and thus directly link them to the equivalent layers in other, nearby local sequences. If the layers
are continuous, this might even work for short distances. Unfortunately, by tracing out our layers laterally, we introduce other problems.

First of all, rock layers do not stay constant in character over long distances. Rocks are a reflection of their
environment of formation. Because there are many different environments accumulating sediments at any given moment
in time, it stands to reason that we will also form many different types of rocks at any given time and that, therefore
rocks of the same age will change in appearance over distance (facies).

Secondly, the same rock layer will only rarely be of the same age in different places. If, for instance, we were to find a deposit of beach sand covering several thousand square miles, it would stand to reason that there was no beach of that
size in the past, just as there is none that size today. A much more likely explanation is that the beach migrated over
time in response to sea level changes, and that the rock layers (deposits) associated with this beach were deposited in
different areas at different times. Reliance on lateral continuity for equivalence of time is more likely to lead to false
interpretations than to correct ones.

To correlate these sequences we need distinctive unique time markers which will enable us not only to tie things of similar ages together, but also to compare and arrange sequences which are not in the same area in chronological order. Occasionally there are events which are distinct, unique, and affect such large areas as to provide us with an almost instantaneous time marker. An example might be a bed of volcanic ash that settled over a large area as a result of a volcanic explosion. Such marker beds, as they are called, do exist, and are extremely useful. Unfortunately, they are rare, random, and non-universal events and therefore have only limited application in a history which spans nearly five billion years.  Only ONE record in all of geologic time has been continuous and ever changing while being common enough to have left traces over virtually the entire earth and that is the record of life, as represented by fossils.
                                     
                                                                                  FOSSILS
Fossils, defined as any evidence of former life, are the remains of organisms. Because organisms have always evolved and changed and because evolutionary changes are always unique and unidirectional, each moment in time is represented by its own unique fauna (animals) and flora (plants). If we can unravel the record of life we can then also unravel the record of time because the two are intertwined.

In the late eighteenth century, two people, an English engineer named William Smith and a French paleontologist, Baron George de Cuvier, discovered independently how useful the fossil sequence is for correlation, even though its cause (evolution) had not yet been figured out. By the mid-nineteenth century, fossils had become universally accepted as tools for correlation, not only on a local, but also on a world-wide basis.

For fossils to be of use for correlation, it is implicit that no two species can be identical either at a given moment in time, or over time. The discovery of millions of species in the fossil record has amply confirmed these assumptions. As can be expected, some species are more useful tools for correlation than others. We call such useful species "index fossils" because they act as time markers. To be an index fossil, the species must be common, widespread geographically, easily recognized, and rapidly evolving. i.e., short-lived as a species from a time standpoint.

Actual correlation between two local sequences, and dating in general, are done by comparing the stage of evolution of the fossils contained in a local sequence to a standard evolutionary sequence which has been worked out on a
world-wide basis by generations of geologists. Comparison of the local fossil to standard index fossils, or, better yet,
index faunas will indicate at what point in the world-wide fossil sequence the local fossil fits. Because there is a
direct equivalency between fossils and time, this will automatically tell at what point in time the fossil existed. Finally,
because the fossil was deposited at the same time as the rock layer, if we know the age of the fossil, we also know the
age of the rock in our local sequence.

Fossils are indeed powerful tools; and in many ways they are the only reliable ones available to a geologist when it comes to unraveling the history of the earth. Still, it bears re-emphasizing that despite the fact that we may be able to ascribe an extremely accurate relative age to a fossil, a layer or an event, the date is a relative date and we cannot know how many thousands or millions of years ago such an event took place. Such determinations are done by using techniques which involve changes which occur at known rates, techniques of absolute dating.