Themes > Science > Paleontology / Paleozoology > About Paleontology / Paleozoology, Generalities > Geologic Time > Local Sequences
The first step in unraveling this sedimentary record involves deciphering the individual local diaries. Geologically speaking, this is called interpreting a local sequence, i.e., figuring out what sequence of events is recorded in a particular local sequence (of rocks) or outcrop (these two words mean the same thing and refer to a place where a sequence of rock layers is exposed to view. For example both the Milhopper and the Grand Canyon are examples of local sequences).
Fortunately, the rules for interpreting local sequences are rather simple. There are five basic principles commonly used (although there are many minor and much more subtle ones). Three of these rules were first clearly articulated by Nicholas Steno in the mid seventeenth century. In capsule form they are:
1)
Principle of Original Horizontality. Because sedimentary deposition is
originally horizontal, any departure from this attitude (such as folding)
must have occurred after deposition and is therefore younger than the
layers themselves.
2) Principle of Lateral Continuity. Sedimentary rocks are deposited as a continuous stratum (layer) and extend in all directions within a basin of deposition. Should they be absent in places within the basin, there must have been events occuring after deposition (such as erosion) which removed these rocks.
3) Principle of Superposition. In any sequence of strata (layers), the oldest layer will be on the bottom, unless some later event, such as when the layers are overturned, has altered this relationship.
The next two principles were stated by Hutton (of uniformitarianism fame) some one hundred years after Steno.
4) Principle of Cross-Cutting Relationships. This principle states that rock bodies or events which cut across other events or bodies are younger than what they cut across. Therefore dikes or unconformities (surfaces of erosion) are younger than the layers they cut across. This principle also applies to faults, where again, the fault will be younger than the layers it cuts across.
5) Principle of Inclusions. If we find fragments of rock of one type incorporated into an other rock type (such as pieces of granite included in a sandstone), these inclusions must be derived from a pre-existing rock and, therefore, are older than the rock they are incorporated into.
Using these simple principles worked out by these two men, geologists can figure out the sequence of events in any local sequence in the world. As much information as we might derive from this, we are still left with a major problem. We now know what happened in this one place, and in what sequence these events took place. But, we still do not know either when this happened, or how this local sequence ties in to other local sequences.
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