Thinking in 3D: Slices through rocks

In sedimentary geology, we often need to visualize the interior of a rock or sediment sample. We use what we can see on the sample’s exterior to imagine what the interior might look like. Although we can’t actually know the exact distribution of clasts within a pebble conglomerate (without destroying it), we can make some good educated guesses about what’s likely, based on what we can see. As an analogy, consider a bowl of fruit salad. Although you can’t know exactly what pieces of fruit you will run into in the middle of the bowl until you eat your way down to them, you can make an educated guess about the relative proportions of different kinds of fruit you will find in the bowl, and also about the distribution of sizes of the pieces of fruit. So, while you can’t draw an exact cross section through the fruit salad, you could draw a representative cross section of it. Clastic sedimentary rocks are similar in that you can infer proportions of clast lithologies and distribution of clast sizes from your observations of the exterior of a sample.

Bowls of rocksLet’s try that. What would a slice through this yummy bowl of ROCKS look like? Sketch it. (Don’t worry about lithologies; focus on the rock shapes and sizes.)

Here’s the same bowl of rocks, but with sand poured in, filling all of the empty spaces. Sketch it again… and think about whether this would be a matrix-supported conglomerate or a clast-supported conglomerate, if it were lithified.

Thomas Hickson, University of St. Thomas, and Ilyse Resnick, University of DelawareSpring, 2015

RockpilesIn geology, we may have to imagine the interiors of individual rocks (like individual pieces of fruit), groups or layers of multiple rocks (like multiple pieces of fruit), or groups of rock fragments (like fruit salad).

Below are some photos of rock piles labeled 1 through 5. If you were to slice these rock piles vertically (you might have to imagine using some kind of magic saw and pretending that the rocks are all glued together in their piles), what would you see? First try gesturing to place your hand in the plane of the cut you’d like. Then visualize how the rocks might look along this cut. Then match each number to a letter on the right.

1. 2. 3. 4. 5.

Thomas Hickson, University of St. Thomas, and Ilyse Resnick, University of DelawareSpring, 2015

Conglomerates and breccias

Above are two photographs of outcrops. Of the rockpiles above, to which are they most similar? Use your visualization and gestures to help infer the shapes of the clasts. Explain your reasoning.

The rock on the left is a conglomerate and the rock on the right is a breccia. Both are made up of coarse-grained sediment in a fine-grained matrix, cemented together. Your response to the question above allows you answer another question: What distinguishes conglomerates from breccias?

Key ideas:- Geology requires penetrative thinking (imagining the interior of an object).- Three tools to help with penetrative thinking are: visualization, gesture, & sketching.- To get good at penetrative thinking, visualization, gesture, and sketching require practice!- The same object oriented different ways will make different shapes when cut by a plane.- A plane oriented different ways will make different shapes when cutting an object.- Sometimes you will not be able to know the exact details of an object’s interior, and

that’s OKAY!- You should use the information available to you, including your geological knowledge

and your observations of a particular feature, to make reasonable inferences. Thomas Hickson, University of St. Thomas, and Ilyse Resnick, University of Delaware

Spring, 2015