The Making of a Field Trip: Planning and Logistics:

By David Brown

This month, my colleagues and I spent time exploring routes for our upcoming field trip planned for later this fall. Such planning is important because the number and timing of field stops have to be coordinated so that everything flows smoothly on the day of the event. The idea is to spend more time looking at geology and less time driving around.

Sometimes this can’t be helped as good outcrops don’t always make good stops. We have already eliminated a few sites because there is simply no place to park. It’s not easy to get 4 vans and 25 people on every location. It can be especially difficult if you’re on the side of a busy road. Safety is always our number one concern; and our second; and our third.

Our first stop was along the west side of Interstate 35 (I-35). We wanted to see the Bromide Formation (the upper part of the Simpson Group), directly across the road from where a dangerous rockfall occurred last summer. Here we found a mirror-image of the rock layers that fell, only these haven’t fallen yet. Time will tell if they do because what we saw were fractured rocks sitting on the top of a hill being undercut by erosion of the finer-grained rocks below. This was the setup that caused the incident across the road last June. It’s not much of a stretch to think it could happen here again.

Getting a closer look with a hand lens.

Our next stop was along U.S. Highway 77, just east of I-35. Here we looked at the boundary, or contact, between the bottom of the Simpson Group and the underlying Arbuckle Group. We found some rock layers with interesting cross bedding, indicating the paleo-environment in which the sediments were originally deposited. This is important because it helps us understand the geologic history of a formation. Some questions are already being raised about the position of this particular contact. But this is what geologists do, they examine the rocks and propose ideas about what they see. We’re expecting a lot more debate on this one.

These rock layers display cross bedding, indicating that tidal forces were present when the sediments were originally deposited.
Acid test
Bubbling after applying diluted hydrochloric acid provides proof of the presence of carbonate materials.

Brittany Pritchett (fellow OGS geologist) and I discovered a wonderful new set of outcrops along a section of road east of the Turner Falls exit on I-35. At least, the outcrops are new to us. Here we found long exposures of the rocks we are studying, and the location is perfect for access and safety. We plan to study this stop in more detail soon.

So, onward we go driving, hiking, and climbing our way through the countryside. If you happen to see some people standing on the side of a road scratching their heads and wrestling with wind-blown maps, give them a honk and a wave. It’s probably just a few confused geologists trying to figure it all out. It will come together. It always does.

An outcrop by U.S. Highway 77, just east of I-35, near the boundary between the bottom of the Simpson Group and the underlying Arbuckle Group.



David Brown Geologist and OPIC Manager

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