Rock Descriptions and Thin Sections

A critical part of developing a geologic map is truly understanding the rock units we observe in the field. The way to do this is by developing rock descriptions. Rock descriptions, which are often initially derived at in the field and further expanded in the lab, are a tricky thing to nail down for a number of reasons. Rocks in some areas may look drastically different from rocks in another, but in reality they are still part of the same macro-package of strata. For example, in the Gladstone quadrangle, I have observed quartzites that vary in crystal size and mica abundance that might possibly indicate they are of different quartzites entirely. However, I can get around this problem by creating thin sections of those hand samples. Thin sections are .03 millimeter thick slabs of rock epoxied onto a rectangular piece of glass for observation under a microscope. These give geologists a close up look of the abundances of certain minerals, micro geologic structures, and the ability to identify any unknown minerals seen in hand samples. What may look like two different rocks in hand sample can be connected as one unit through the aid of a thin section, and vice versa.

The Smith River Allochthon has a very specific metamorphic grade and mineralogical profile. Creating thin sections of the many ‘bland’ phyllites and schists that I’ve encountered gives me the ability to see if they match any published thin section descriptions of the SRA from farther south in Virginia.


  1. Thank you for sharing how thin sections work. I’ve been loosely following your group’s progress throughout the summer through your Facebook page and have dabbled in some geology but I haven’t taken any of the core geology classes yet. How do you go about making a thin section? Do you make them in the field or is this something you do back in the lab? Also what does it mean for a phyllite or schist to be “bland”? Thanks!

  2. Of course, thanks for reading! A thin section is made by a third party distributor, though the geology department does have some obsolete equipment that was once used to create them on site. In theory we could make them in the lab, but never in the field. Thin sections are, for all intents and purposes, lab tools. For this project’s purpose, we use Spectrum Petrographics, a company Chuck has used in the past.

    As for a “bland” schist or phyllite, I really mean that there are no ‘interesting’ minerals (in this case, minerals indicative of higher metamorphic conditions). Much of what we see in the field is a low-grade, grey metamorphic rock with very small crystal sizes. Nothing to write home about, that’s for sure!