Relative Time

OK, so you think you’ve grasped the concept of geologic time. You have studied the geologic time scale and know the different geologic time periods like the back of your hand. But when you head out to look at the rocks and you find a huge jumble of rocks, some tilted, some horizontal, some intruded by igneous rocks, and all of it slightly confusing. How does a geologist interpret these complex events and place them in their proper chronological order and finally place the rocks within the confines of the geologic time scale?

Geologists have two methods to help them interpret the age of the rocks. The use of absolute time concerns itself with measuring the exact age of a rock or fossil to determine its placement in the time scale. The other method, and one geologists are more likely to use, is relative time, placing the rocks within a relative sequence of events. We will concern ourselves with relative time in this article, and cover absolute time in the next article.

First, let us consider the following diagram. It shows a hypothetical geological section. In it, consider each of the colored blocks as separate geological formations. Using this section and the information below, try to interpret the sequence of events that occurred to create this sequence.

There are three principles that are used to interpret a specific section. I have covered two of these principles in my earlier article on Stratigraphy, but I will cover them again here. The first is the principle of Original Horizontality. Nicolaus Steno first developed this principle and it states that sediments on a solid base must have been deposited horizontally. Looking at our hypothetical geological section we can see that several of the formations are tilted. Using this principle we can draw the conclusion that these beds were originally laid down in a horizontal layer and were then tilted at a later time.

The second principle, also postulated by Nicolaus Steno, is the law of Superposition and states that each layer of sediment must have been deposited onto a solid layer, and therefore each successive layer is necessarily younger than the layer below it. Using this principle, we can determine that the uppermost horizontal layer is the youngest layer in the sequence. Using the principle of original horizontality and un-tilting the tilted rocks, we can see that the purple formation is the youngest formation among the tilted rocks, the pink formation the oldest.

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