Self-Check: Relative Dating Practice

Now it is time to be a detective and see if you can tell the story of the geologic history of this area.

The following features A through F represent:

A  Folded sedimentary rock strata
B  A magma body that welled up into the crust
C  A sedimentary rock layer
D  A dike
E  A sedimentary rock layer
F  A fault

Using the principles of relative dating, can you place features, A through F, in chronological order from oldest to youngest?

Oldest to Youngest	Explanation
A	These folded rock strata are the oldest features. Features B, D, and F cut through them, so you know by cross-cutting relations that B, D, and F are younger than A. Features C and E are on top of them, so you know by superposition that C and E are younger than A.
B	This is a magma body that welled up into the crust. It has to be younger than A because it cuts through A. But it must be older than D and F because D and F cut through it. By superposition, you know that it is older than C and E.
C	This rock layer must be younger than A by superposition. However, it is older than D because D cuts through it.
D	By the principle of cross-cutting relations you know that this dike is younger than A, B, and C because it cuts through them. By superposition, it is older than E because it lies below E.
E	This sedimentary rock layer is younger than A through D because it lies over the other features; thus by superposition, you know it is younger than them. It is older than F because F cuts through E; by cross-cutting relations you know that F is younger than E.
F	This fault line is the youngest feature. It cuts through features A, B, C, and E so by cross-cutting relations it must be younger than them. Since it cuts through E but D does not, then F must also be younger than D.

 

What does the principle of original horizontality tell you about rock layer A?
 

This layer was originally horizontal. It must have been tilted and deformed by tectonic movement. You know that this action took place after the rock layer was formed but before events such as C, D, E, and F.