The record of past events which took place on the surface of the earth is entombed within the rocks. Although some information can be gleaned from igneous and metamorphic rocks, these rock types mainly record events which took place at depth, and help us unravel plate tectonic events. Most of the information available about surface events is linked to those rocks which formed at the surface, sedimentary rocks. Covering some 70% of the earth's surface, these rocks have two characteristics which are of central importance for time considerations. Firstly they are layered, and secondly they contain fossils. 

The question is, how can geologists decipher the available record? To use an analogy, the record we have is the equivalent of loose-leaf rock diaries, utterly lacking dates, scattered all over the earth, whose pages have been subjected to the ravages of erosion. Moreover, the story was written in a language which we can understand only if we understand the processes we can observe going on today. 

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 outcrop (a place where rock layers are exposed to view). 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.