I have a homework question that I'm stuck on, I have attempted interpreting this map but am not sure if I have to correct interpretation.

Geological map

  1. What is the youngest/oldest rock in the geological history of this map?

By law of superposition, oldest: limestone, youngest: mudstone. For this question, I'm used to looking at the law of superposition to determine age but in this diagram the contours don't tell me anything. Did I get these right?

  1. What type of fold is shown on the map?


  1. Give a brief geological history of map.

This is the one I need most help in. Magma intrusion at fault lines? Maybe some mudstone intrusion over limestone?.

Again, please tell me if these answers are wrong.


2 Answers 2


1. What is the youngest/oldest rock in the geological history of this map.

Your approach is the right one but isn't the limestone underlain by something? Drawing the AB cross section would be of great help here, did you manage to do that?

2. What type of fold is shown on the map.

What is the definition of a syncline and does your interpretation agree with your answer to question one? Are there any legend symbols on the map which would be used to indicate a fold?

3. Brief geological history of map

Putting all the lithologies and the deformation (Question 2) in right order would be a big part of that. After that maybe you have some ideas about a succession of claystone, limestone and mudstone

  • $\begingroup$ I haven't done the cross section yet. So the fold axis on the map shows that the folds are going inwards right?. Unless I'm getting the information on the lecture slides mixed up, shouldn't the first layer be the youngest?. i can see that it is a syncline now though. $\endgroup$ Commented Oct 1, 2015 at 9:51
  • $\begingroup$ Hi again, if I draw an A-B cross section of the map, is it possible you could give me feedback. I'm drawing it atm but i'm not sure what to do with the 200m-300m dashed line contours $\endgroup$ Commented Oct 1, 2015 at 10:23
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    $\begingroup$ You should have learned in an earlier lesson how to translate the contour lines into a profile, maybe have a look a that again. They are not so important though, they are only there to tell you the dip of the basalt intrusions. All the rest should be in Gordons answer $\endgroup$
    – wienein
    Commented Oct 1, 2015 at 10:37
  • $\begingroup$ so wait, should the basalt dyke be at the angle shown on the map, not a vertical intrusion? $\endgroup$ Commented Oct 1, 2015 at 11:37
  • $\begingroup$ They are vertical, but we could not be certain about that without the information from the contours. $\endgroup$
    – wienein
    Commented Oct 1, 2015 at 12:17

From the dips it is obvious that the stratigraphic structure is a syncline, in which case you correctly deduce that the youngest layered rock is the mudstone. But look carefully. The dips on either side of the fold axis are more than at right angles to each other, indicating an open fold of about a 110 degree deformation. Now consider the basalt dykes: basalt is normally intruded as sheets along planar fractures. It is obvious that these dykes have not been deformed, or they would be curved. Therefore they are post-deformational, and are therefore the youngest rocks on the map.

  • $\begingroup$ Ok, I kind of get how to find the youngest now, but I took a guess at the oldest and said it was limestone. For the claystone, am I right in saying that it's not a part of the fold?. $\endgroup$ Commented Oct 1, 2015 at 9:55
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    $\begingroup$ No, the claystone is very much part of the fold. Note the similarity of dips between the limestone and claystone on both sides of the axis. Hence the claystone is structurally the lowest, and hence the oldest. $\endgroup$ Commented Oct 1, 2015 at 10:15
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    $\begingroup$ Your cross section is pretty much correct. The dips of the basalt, if any, are not given so there is nothing to say if they are vertical or not. Though probaly if they were not close to vertical they would appear as some curvature on the hillsides. I think the question is mainly about deducing their relative age. You might want to sketch in the apparent fold axis on the original map. It is fairly simple, as there seems to be no plunge either way. $\endgroup$ Commented Oct 1, 2015 at 12:17

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