Into the Third Dimension: using Google Maps to know what’s underground

Much of the earth’s surface is covered by sedimentary rocks. These form as sediment settles on the surface. As the types of sediment change – sand to mud to sand again – different layers are formed, some hard some soft. The patterns these layers make are responsible for some of the most interesting Great Geology in Google Earth.

Sedimentary layers start off flat1 but as plates collide and squash, they may be folded, like pushing the edge of a rug. The resulting 3-dimensional structures are later eroded and brought to the surface – itself a 3-D structure. The 2-D lines we see on the aerial images below are formed by the intersections of these different 3-D structures. This can make interpreting them a little difficult, as we’ll see.

Compare and contrast the next two images. First look at these smooth lines from Mexico.

Versus these zig-zag ones from Argentina.

Both of these patterns involve sedimentary rocks, but the causes of the wavy lines are very different. The secret is to work out the shape of land surface and then infer the shape of the sedimentary layers. Rivers are our friends here. In Argentina there is a clear relationship between them and the lines the layers are making on the ground. Every zig has a river in it and every zag is a little hill in between. The sedimentary layers are pretty much flat and the pattern of lines is caused by the shape of the land surface. Imagine cutting a wedge out of a layered cake – this is what it would look like.

In Mexico the pattern is mostly due to folding of the sediments. The beautiful curves and swoops are due to flat layers having been slowly buckled as the earth’s plates rearranged themselves.

Analysing these shapes and making sense of them is bread-and-butter for geologists. A bed-rock part of any geological education. Typically we use maps, but in desert areas photos tell everything we need to know.

One important trick geologists learn is to create cross-sections, drawing a slice through the earth to show how the folded rocks continue underground. One of the many types of sedimentary layer is a coal-seam, so you can see how this is not a purely academic exercise.

A good geological map will have symbols showing how many degrees tilt the layers have and in which direction they are pointing down. Without these we can’t do a proper cross-section. But using our friends the rivers and streams we can still tell a lot.

Here’s a nice fold. Imagine one of the layers is rich in valuable unobtanium and you want to mine it2. You can trace a line where the edge of it is, but which side of the line is the rest on? Think of it another way, are we looking down at an arch with the top sliced of (an antiform) or a basin (a synform)3 Should you sink your unobtanium mine-shaft in the middle of the fold or around the edge? If you get it wrong, you’ll choose the part where the valuable layer has already been eroded away from.

dipToTheNorthLook at the rivers (streams/creeks) that cross the layers on the top side of the fold. Notice a pattern? Every time the stream cross the layers, it makes a little V-shape, with the sharp bit of the V pointing North. A stream bed is always lower than its surroundings so it gives us a glimpse into the third dimension. Cut into the layer and it’s edge moves north – it’s deeper the further north you – it’s dipping to the north – it’s a tilted sheet that disappears under the ground to the north.

dipToTheSouthWe can check if we’re right because this a fold. For this trick to work the south side of the fold should have the opposite pattern. We are looking at a breached arch and the southern side should have the opposite pattern. It’s harder to see on this side (probably because the beds are tilted more nearly vertically and the effect is smaller) but indeed, our little V-shapes point the other way.

This 3-D stuff is hard work. Yet it’s something geologists have to be good at (maybe there’s some link with the strong anecdotal evidence that they tend to be left-handed). If you need some help, some of these Google Maps have good photos associated to help you get another view of the structures. Alternatively this post makes the same link.

Let’s leave you with some sheer aesthetic pleasure. Some totally flat layers turned into beautiful patterns by erosion.

Notes:
1. Well, flat-ish. Layers may well slope gently, capturing the slope of an ancient sea bed or desert dune
2. And let’s assume you somehow want to site the mine without first visiting the area. Perhaps you are a super-villain unwilling to leave your volcano lair and you don’t trust your henchmen
3. You may be more familiar with the terms anticline and syncline. These are respectively where the oldest rocks are in the middle of the fold, or round the outside. Most times an antiform is an anticline, but not if the rocks are upside down.
Categories: Great Geology in Google earth, tectonics

Comments (2)

  1. Matt Hall says:

    Nice post, and nice collection of images. But even with your footnote, I don’t think one can say:

    Sedimentary layers start off flat…

    I mean, I guess it depends what you mean by ‘flattish’, but it’s still not really true. Sedimentary layers dip at lots of scales and for lots of reasons. What was once called the ‘Principle of original horizontality’ is now generally held to be ‘not useful’. The question came up on Earth Science Stack Exchange a while back and I gave some links and so on there.

    Anyway, just a little nitpick. It’s still a nice post :)

    • Metageologist says:

      Hi Matt,
      That’s fair comment, thanks for the extra detail.

      I think you and I are showing the different perspectives of our bits of Earth Sciences when we use the word ‘flattish’. As a recovering structural geologist there is still a part of me that regards rocks that aren’t actually upside down as being only gently folded and thinks that sedimentary structures are best used as way-up indicators.
      And for you as someone working with sedimentary rocks, the fact that they were never really flat in the first place is quite rightly of great interest.It just shows that no geologist views the same rocks in the same way.

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