Lab 10 Exercises
The ability to visualize strata in three-dimensions is a spatial skill that is fundamental to understanding geology in the real world. For some, this skill comes easily. Bur for others, spatial thinking and visualization can be difficult concepts, especially at first. Regardless of which category of student you fall into, while these skills may come more naturally to some, they can be taught and will improve with practice! To practice visualizing in three-dimensions, think about the difference between how an object looks from above (in plan view), and how a slice through that object would look (in cross-section).
Spatial thinking and cross-sections
Imagine an apple. How would the apple look in plan view?
Now imagine slicing the apple in half, along a straight line. If you pull the two halves apart to examine the internal structure of the apple, you are looking at a cross-section. The photographs below show how the apple looks in plan view (left), and in cross-section (right). The dashed white line is the line of cross-section, from X to Y.
For more practice, imagine and sketch what the cross-section of the following objects would be:
- an orange
- a lemon cut in half from end to end
- a round loaf of sourdough bread, cut into slices
- a pyramid with a square base, cut along a diagonal line from corner to corner
Today you will use printed copies of the block models in Appendix 5 to explore geologic structures. Recall that the top surface of a block model represents the plan view of the Earth’s surface, and the four sides represent four cross-sectional views down into the subsurface. On each of the six block models below, the geology drawn on the south- and west-facing sides shows the geology of the subsurface. This will give you some clues about the attitude (dip and dip direction) of the formations, as well as any structures present. Examine each of the block models, complete the missing sides, and use your completed model to answer the questions below. Unless otherwise stated, assume all the formations below represent sedimentary rocks.
For some of the models you will be asked to write a point-form geologic history based on what you observe in the model. This is an excellent way to review the relative dating and geologic time concepts from Lab 7. For example, try writing a point-form geologic history for Block Model 1 that shows a sequence of Cambrian (brown), Ordovician (light grey), Silurian (cream), Devonian (blue grey), and Mississippian (orange) formations that are dipping to the east, as well as a Cretaceous gabbro dyke (dark brown).
A point-form geologic history for Block Model 1 would look something like this:
Youngest (most recent)
- Erosion +/- uplift to present
- Intrusion of Cretaceous gabbro dyke (cross-cutting relationships)
- Tilting of the Cambrian to Mississippian formations toward the east (original horizontality)
- Deposition and lithification of the Mississippian formation (superposition)
- Deposition and lit