In Lecture 9, we worked out the sequence of events apparent from the upper of three cross sections. This page gives the sequences for the lower two. You'll get far more out of this exercise if you try to work out the sequences of events for yourself and then check your lists against the lists below.

Middle cross section:

Latest: Intrusion and extrusion of B
Erosion at T
Intrusion of R
Intrusion of U
Deposition of Z
Deposition of M
Erosion at L
Faulting at J
Deposition of P
Deposition of H
Deposition of Q
Erosion at S
Intrusion of A
Intrusion of D
Earliest: Intrusion of N

Points worth noting:
We can determine the age relation of A to B, even though they don't cross-cut each other.
Some thick mass of rock older than N had to exist, and N was intruded into it, and then all that overlying rock was eroded.

Bottom cross section:

Latest: Deposition of T
Erosion at I  
Faulting at J   Faulting at M follows after U; its relation to J is uncertain.
Deposition of U
Depositon of A
Erosion at P
Faullting at D
Intrusion of S
Faulting at Q and R
Deposition of G
Deposition of B
Deposition of Z
Deposition of X
Deposition of C
Earliest: Deposition of H

Note that this cross section records two phases or tectonic history. The first, in which normal faults Q, R, and D formed, was a period of extension. The second, in which reverse faults J and M formed, was a period of compression.


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UGA Geology Department web page