The Principle of Cross-cutting Relationships

Themes > Science > Earth Sciences > Geology > About Geology, Generalities > Principles of Geology > Other Basic Geologic Principles > The Principle of Cross-cutting Relationships

Where a fault cuts across a sequence of sedimentary rock, the relative ages of the fault and the sedimentary sequence can be determined. The fault is younger than the rocks it cuts. The sedimentary rocks are older than the fault which cuts them, because they had to be there first, before they could be faulted.

When observing a faulted sequence of sedimentary strata, always look to see how the beds on either side of the fault have been displaced. You might be able to locate a "key bed" which has been offset by the fault. If so, you will be able to determine the type of fault (normal fault, reverse fault, etc.).

(1) Normal fault (2) Reverse fault

Examples of faults to illustrate cross-cutting relationships.
(1) Unit A is the oldest, followed by B and C. Fault D is the youngest.
(2) Unit A is the oldest, followed by B and C. Fault D is younger than C, but older than unit E.

Information provided by: http://www.gpc.peachnet.edu

(1) Normal fault	(2) Reverse fault
Examples of faults to illustrate cross-cutting relationships. (1) Unit A is the oldest, followed by B and C. Fault D is the youngest. (2) Unit A is the oldest, followed by B and C. Fault D is younger than C, but older than unit E.
Information provided by: http://www.gpc.peachnet.edu