17 April 2012

Strained Antietam Formation sandstone

Posted by Callan Bentley

I collected this sample the weekend before last on the Blue Ridge Thrust Fault field trip led by Alan Pitts. It’s a chunk of the Antietam Formation quartz sandstone, a Cambrian beach deposit. The face we photographed measures 15 cm by 12 cm.


It definitely looks best in full screen mode, so please feel encouraged to click through and explore it a bit. You’ll notice some great subtleties once you zoom in. To illustrated these, here is a  screenshot I took with my iPad*, followed by (crudely) annotated versions, made with the “Whiteboard” app:

Long axes and short axes of the strain ellipse drawn in:

Outlining the edges of fused masses of sand grains:

Highlighting the empty space along strike of the long axes of the grains:

These images show that this sample experienced some degree of pressure solution (“p sol,” as I’ve learned to call it), and was squeezed (relative to this photo) from the upper left towards the lower right. This caused the rock to shorten in that direction, and the grains to pack closer together (even “suturing” into their neighbors) from the upper left towards the lower right. In contrast, the upper-right-to-lower-left direction is exemplified by both the long axes of the grains, and empty space in the “pressure shadow” of resistant grains. In some places in the main GigaPan, you can even see some white fringes in the X-axis direction that may be pressure fringes of new mineral deposits.

Pretty sweet little sample. The weathering with individual grains standing out in high relief (minimal cement) was what originally caught my eye, but then the preferred alignment was what made me decide to keep it, and ask Robin to subject it to the M.A.G.I.C. macro photography treatment last week. Not only is the sample itself cool, but I think Robin did an amazing job with the image. This is one of my favorite macro GigaPans that our group has produced so far.


* GigaPans are Flash-based, so they don’t work on regular web browsers in the iPad. There is an app for viewing them on the iPad, but it had some problems, and is currently being re-engineered. I’m assured that people are working on it. In the meantime, the “work-around” way to view GigaPans on the iPad is with the following URL (for this specific GigaPan):


For other GigaPans, just copy the unique I.D. number of the image and replace the final part of the URL (after the “equals” sign) with that number. Hours of fun geologic exploration await!