Friday Rocks #26: Drag folds and pressure solution in limestone


Drag folds in limestone. Pressure solution in the fault plane.
Photo by Tim Sherry

Here is a fault in limestone. The drag folds indicate a top to the left thrust motion. In the fault plane there is evidence for pressure solution (brown-tan thin lines cutting a diagonal through the middle of the photo), a type of plastic deformation. Pressure solution involves the dissolution, transport, and re-precipitation of minerals from areas of high stress, such as grain-to-grain contacts to areas of low stress.

These photos were taken on “Six Mile Fold” outside of Boulder, CO.

Drag folds and pressure solution.
Photo by Tim Sherry

Friday Rocks #25 Fishmouth Structure

Photo by Christie Rowe

This week’s Friday Rocks comes from Christie D. Rowe. You can follow her on Twitter at @KeepItRheol

These are at Two Lights state park in Portland ME.
Faulting was accommodated by a combination of brittle fracture and plastic elongation of the already lineated metasedimentary rock.  This is displayed really well in the photo with te red bottle cap – at the end of a fault, instead of an array of tensional faults, the foliation is sucked in toward the fault tip in what Swanson calls a “fish mouth” structure.  Haven’t seen it anywhere else so I can’t tell you if that’s a real term or not, but pretty amazing anyway.
Photo by Christie Rowe
Photo by Christie Rowe

Do you have a sweet outcrop photo? Submit it with a short description / interpretation to be featured on Friday Rocks!

What’s in my in the field photo kit?


Self portrait via Brunton
Self portrait via Brunton

Field sciences such as geology, biology, anthropology, paleontology rely heavily on photographs to present evidence and convey the story that in the geologists case, is read and interpreted from the outcrop. Nowadays the camera is just as important as the hammer in the geologist’s toolkit.

In terms of cameras there are so many choices for what tools to take into the field. No set up is necessarily better than another… “the best camera is the one that’s with you”. I’d like to share with you the tools I bring into the field.

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Camera – Nikon D7000
Like every photographer just starting out, I had to make a decision on what brand to go with. I had long discussions with my dad, also a photographer about what camera to start with. Well, my dad is a Canon guy, but my grandfather was a Nikon guy. As a scientist that didn’t really get me closer to making a decision.

My dad and I researched the different camera packages on sale, some Nikon some Canon. When I had the models narrowed down I started digging into the technical specs. I primarily used the website Digital Photography Preview to dig into details. I also recommend Ken Rockwell’s camera and lens reviews.Two finalists remained, a Canon and the Nikon D7000.


I decided to go with the D7000 because it had a magnesium-alloy body whereas the Canon had a plastic body. I wanted a camera that could stand up to the moderate to heavy hiking and traverses involved with field geology. The downside is that protection come with some added weight. Plus it came packaged with a great lens…

Lens – Nikon 18-200mm VR II AF-S Zoom Nikor ED, takes 72mm filters
This lens is a workhorse. I absolutely love it. Yes it is heavy, but the wide – telephoto range means that you only carry one lens, so no changing out lenses. The 18mm isn’t quite as wide as single purpose wide angle lenses, but with the help of photo stitching software it is possible to make some great panoramas and landscapes. On the other side of the range, the telephoto is extremely useful for getting close to the outcrop details. And for spontaneous portrait photography of animals and field assistants.

As for the lens hood. I almost never have it on, I keep it in my bag in case of rain.

Dolomitization front through interbedded limestone and shale
Dolomitization front through interbedded limestone and shale

Lens Filter
Always use a lens filter. The cost is small compared to the cost of the lens. I recently switched from the Nikon clear filter, which has gotten quite beat over three years, to a UV filter. Haven’t had too much of a chance to use it so I’ll report back on any differences.
I take two Nikon EN-EL15 batteries into the field with me. Always good to have a backup. I’ve switched out either battery for three years and they still hold a good charge.

Lens wipes

I carry a handful of these for keeping the lens clean. I travel to dusty, sandy, dirty places. These are a must to get clean photos.

Memory Card
These have gotten so cheap that it really is ridiculous. There’s no excuse for running out of pictures on your camera. I use two 16 GB in my D7000. If I’m out for a very long time the second SD slot is overflow. In other situations the second SD slot acts as a backup. I take that SD card out and keep it hidden away from the camera in case my camera is stolen.

I also recently got a little usb-miniSD reader so I can transfer photos from my camera to my phone. So I carry a couple miniSD with SD adapters now too.

Slickenlines on the Naukluft Thrust
Slickenlines on the Naukluft Thrust

Get a case that will protect your camera and lens when you’re on the move. Just get one that fits your set up. I use a Lowepro Toploader Zoom 50 AW. Don’t bother with the strap, it broke pretty fast for me. I keep it at the top of my backpack for quick access. Otherwise my camera is on my neck or shoulder.

I use the standard Nikon strap that came with my camera. It is getting worn down so I’ll likely replace it soon.

Parting thoughts
Again, there is no right answer in terms of cameras and gear for the field. What suits me might not suit the next geologist.

What camera set up is in your field kit? Let us know in the comments!

Friday Rocks #24: Tourmaline in tonalite

Tourmaline in Tonalite
Tourmaline in Tonalite

For this week’s #FridayRocks we have tourmaline in tonalite. This is a close up of the Mega-Boudin from Friday Rocks #22. Here tourmaline forms clusters at the margin of the foliated tonalite. I didn’t look with a handlens or at a thin section, but I suspect that the foliated zone is comprised of deformed tourmaline.

Call for submissions! I am going away for a trip and am looking for #FridayRocks submissions. Do you have some cool outcrop photos? Please write a brief description / interpretation to send along with the photo timothyjsherry [at] upsection [dot] com

Tourmaline close up
Tourmaline close up
Tourmaline clusters localized by strain
Tourmaline clusters localized by strain

Friday Rocks #23: Reactivated Thrust Faults

Reactivated thrust fault in Niobrara Formation, Colorado
Reactivated thrust fault in Niobrara Formation, Colorado. Photo by Tim Sherry


Today’s Friday Rocks is a reactivated thrust fault in Niobrara Formation, Colorado. The red/orange marker beds can be misleading as not all of them are in the camera frame. Look instead at the drag folds in beds near the fault zone. Yes, it is complex, there was a reactivation along this same fault. Infact, two sets of slickenlines with different orientations are found on the fault.

Here’s a photo without scale:

Reactivated Thrusts, Niobrara Formation, CO.
Photo by Tim Sherry

Do you have a cool outcrop photo? Submit it for a Friday Rocks! Email the photo with a short description / interpretation to contact[at]

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New Site

The website of Timothy J Sherry

As you can tell I’m giving Upsection a much needed make over. Please be patient while I get the bugs ironed out during the transition. Any thoughts or comments? Leave them below or shoot me an email. Thanks – Tim