What does it mean to measure a rock? I know this is what you have been thinking about ever since you subscribed to this blog several months ago. And to satisfy your insatiable hunger for geology-knowledge I, Haley, will present a tutorial on measuring folds in rocks.
WHAT?! Rocks have folds????
In fact! Yes! They do. Have a look:
This is a wonderful fold in three dimensional view. Peter and I found it near Singing Sands Beach – the southernmost cove on Small Point. Now, you can see all the layers shaped like half a cinnamon roll in the foreground and you should be able to see the continuation of those layers in a cylindrical fashion as you move your eyes to the background. Here is a little help (Yay, Powerpoint!):
Now, for the tricky part. You need to visualize the fold three dimensionally. It isn’t just a rock face you are looking at. Those layers extend toward the back of the picture. Similar to this cylinder:
So now you can see the fold, but how do you MEASURE it?
Be patient, young grasshopper and I shall tell all…
Geologists like to break things down into planes. Yes. Think Algebra. I know it hurts, but go with me:
Here is a plane that I can see pretty easily. Now, of course you are saying, “But you just made something curvy flat! You can’t do that!”
If you measure enough planes in a fold (three to be precise) you can later redraw the measurements and come up with a fold that is pretty darn close to this one. And here is where you break out the algebra. (Ahhhh!) You need to visualize this plane in a coordinate system of three dimensional space. Instead of using ordinate and abscissa axis though, we geologists simplify and use a cardinal direction! Yay! Only one number! You need to know the bearing of the plane.
In this photograph, you can see I have placed a north arrow on top of the plane. This way you can make a guess as to what the bearing of the limb is. I would say it is about NNE. In the field, we would use our compass to get an exact number for the bearing. This would probably be about 20° on a compass.
Now we only need one other measurement for this plane… Now, we need a third axis (like the z-axis in the cartesian coordinate system) in order to understand completely its orientation in space because we need to measure how vertical the plane is. Our measurement of the vertical incline of a plane is based on a concept and system that is the same no matter where you go in the world (unlike the cartesian system): gravity. We just need to measure the steepness of the plane to record its orientation in the third dimension. This steepness is measured as an angle from a flat horizontal line (a horizontal line, which is always exactly perpendicular to gravity).
Powerpoint has this nifty feature that snaps a line to the horizontal of the page. What comes on a compass is just as fun: we have compasses with levels so that we can find a horizontal line and measure the angle with ease.
Now, almost there. We measured one plane, we have two more.
The next is the other limb of the fold, which is essentially the above plane, just flipped to the other side of the layers. I think it should be fairly simple to visualize and I was running out of patience with Powerpoint, so you’ll have to use your imagination.
The trickiest thing to measure and visualize is the plane that cuts the folds in half and describes where the two limbs meet. Here is what that plane would look like:
You would use the same technique of cardinal direction and steepness to measure this plane. Once all the planes are measured, you have enough information to redraw the fold, or perhaps more importantly compare it to other folds and folding patterns in your area. The measurements of “cardinal direction and steepness” are known to geologists as “strike and dip.”
(Side note: When Peter and I were measuring today we met someone on the beach who asked us, “Are you measuring strike and dip?” We stared back in happy surprise to the man tanning on the beach and said, “Yes!” He told us he was a geo major from Lehigh University. As we told him the scope of our summer field work he reminisced about geology field camp at Lehigh. Somewhat of an “initiation camp” into the geology cult, he described. Me thinks we need some kind of “initiation camp” at Bates… Harharhar.)
The size of folds, the frequency of folds, the shape of folds, and the orientations of folds can help a geologist come up with part of the story of the rocks you are looking at. It can also help explain the dynamic movement of rock miles below the crust – something you could never see with the naked eye.