Developing a local stratigraphy
- Understand how information from several exposures can be combined into a composite stratigraphic section
- Constructing a stratigraphic section of an outcrop
- Description of hand specimens and outcrops
- Sketching outcrops
Context for Use
You can fashion between one and four or more field labs dealing with local stratigraphy, depending on the complexity of the stratigraphy (number of units), spacing of the outcrops, and goals of the course. You can use a glacial geology lab e.g. this one to complement these labs. You can also design the class so that a final project is a geologic history that students compile, based on observations and interpretations they have made in the field labs. See Assembling a geologic history for ideas about how to construct such an assignment.
Depending on distance to outcrops and the students' previous experience describing rocks in hand specimen, you should allow between one and three hours of outcrop time for each of the labs.
- Hammer and/or shovels/trowels (depending on the nature of the material)
- 30 or 50-meter tapes
- Topographic maps to get locations and elevations of outcrops
- Altimeter (optional, but can be useful in regions of gently dipping strata; however, see caveat below)
Teaching Notes and Tips
Each of the stratigraphy labs will probably take one to three hours, not counting driving time, depending on the height of the stratigraphic section to be described and the complexity of the rock types. It is possible to shorten the required time, if students are asked to describe only certain rock features (such as the direction of cross-bedding) and not others. Once you've toured the regional geology, it's generally obvious what rock units are good for what kinds of descriptions and measurements. For instance, in southern Minnesota, one of the Paleozoic sandstones (the Jordan Formation) has abundant cross-beds in part of its exposed section while the other Paleozoic sandstone (the St. Peter) does not. You can also group nearby outcrops into a single lab.
It is helpful to have students think about stratigraphic correlation at each outcrop. If possible, start with an exposure that has more than one rock unit present, even if the minor units are barely thick enough to recognize, let alone describe. Also, be aware that even in areas with horizontally bedded rocks, the rocks are seldom "flat" enough to appear at the same elevations over several kilometers of exposure. No matter how careful you and the students are about collecting elevation information (from topographic maps and altimeters) and bedding orientation information, you'll find that elevation is seldom a perfect correlation technique. So engage the students in figuring out how, in the absence of consistent elevations, one might correlate rocks at different locations.
If you are able to do more than one field lab centered on local stratigraphy, each of the write-ups can focus on a different skill that can then be the basis for assessment. These might include: outcrop description and sketch, stratigraphic section of an outcrop with several rock types exposed, fence diagram, detailed stratigraphy of a short section of complex sediments, etc.