New Triassic site

Geologists have an interesting symbiotic relationship with highway departments. A knowledge of local geology is necessary for building a road or bridge properly. At the same time, road construction often exposes rocks that otherwise would be inaccessible for study.

Because road construction is time-sensitive, we don’t always get a lot of warning when interesting rocks are exposed. This morning we received a call from Dave Woolley, a geologist with the Virginia Department of Transportation, that they had uncovered some Triassic rocks in Pittsylvania County while excavating to install new bridge pilings on a local road. Richard Hoffman (VMNH Director of Research), Jim Beard (Director of Earth Sciences), and I rushed out to the site to see what they had found.

The Triassic rocks are the black ones exposed in the floor of the stream (VDOT temporarily diverted the stream for the excavation). Part of the fun of geology–I got to climb down into the mud to measure the orientation of the rocks (important for mapping the deposits.)

VDOT thoughtfully made a pile of the material off to the side, so that we could look over it and load it in our truck. At a glance it’s not much to look at, but the rock is very carbon-rich, essentially coal in places, and so likely to contain plant fossils. We brought a truckload back to the museum where we can clean it and examine it more closely.

Even without cleaning, a quick look at the rocks reveals tiny pieces of plant material, as indicated by the arrows:

It’s possible, but unlikely, that we may recover more visually impressive specimens from these samples. But they have already proven useful, in giving us another bit of information about the extent of the Triassic rocks in the Danville basin.

Thanks to Dave Woolley at VDOT for contacting us about this site, and making it accessible to the museum.

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3 Responses to New Triassic site

  1. Doug says:

    Ya never know until you crack the rocks open. My dad gave me a rock that he had found on the base he works at. I cracked it open an there was a strand of seaweed inside. It’s really cool.

  2. Aydin says:

    Excuse my ignorance, but how does a geologist look at a rock & tell that it is a Triassic rock or whatever.

  3. Alton Dooley says:

    I admit I’m hedging a bit on these particular rocks–we already knew there were Triassic rocks in this general area, so we were expecting these.

    Of course, that just moves the question back–how did we know those other rocks were Triassic?

    The single most useful method for getting the age of sedimentary rocks is by looking at the fossils they contain. Each different time has its own unique set of fossils, so if you identify that set, you know your time period. The field of biostratigraphy is primarily concerned with developing and testing time scales based on fossils.

    There are several other techniques as well, including looking at the relative percentages of certain stable isotopes, and looking at the pattern of magnetic polarity in the rock.

    These are all relative time scales, that can tell you the sequence of past events, but can’t directly tell you their age (in terms of millions of years ago.) For that, the most important method is dating based on the decay of radioactive isotopes. This technique is mostly used on igneous rocks, and can give a very accurate date. The other time scales (like ones based on biostratigraphy) are calibrated using radioisotope dates.

    For the Triassic rocks in the eastern United States, we have a fortunate situation in that there were erupting volcanoes in the same time and place that the sedimentary rocks were forming (the volcanoes are a result of the same rifting process.) So our dates on the Triassic rocks in this area are pretty solid.

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