Rumeli Hisarı

Right after I got to Istanbul on this most recent trip, I took a taxi from my hotel down to the Bosphorus, to check out the Rumeli Hisarı, a fort complex built in 1452 by Sultan Mehmet the II in anticipation of the following year’s siege of Constantinople. It’s constructed at the narrowest point on the Bosphorus (660 m wide), with the aim of controlling boat traffic coming from the Black Sea. This narrow spot is today where they have the second of two bridges spanning the Bosphorus. It looks like this:

It’s in Europe; that’s Asia on the far right of the photo. A few more shots of the fortress’s pattern of towers and interconnecting walls:

Inside, I was pleased to note the variety of building stones. Here’s a nice porphyritic andesite which was a common constituent of the walls:

And a folded limestone:

Here are some yellowish blocks that are weathering away faster than the mortar which holds them in place. There is a Turkish 1-lira coin in front of the dark block near the center, to provide a sense of scale:

Here’s a similar phenomenon playing out with some bricks used to make an archway, except here the mortar is the more rapidly weathering component:

Check out this slab of brick… it’s got a curious adornment:

Zoomed in to show this detail:

Dog prints! Sometime a long time ago, maybe more than 500 years ago, a brick maker put out slabs of clay to dry, and some long-dead dog walked across it. The dog’s footprints are a kind of “historical trace fossil” that was then incorporated into this ancient structure.

Visiting the Rumeli Hisarı was a pleasant experience. I walked down along the Bosphorus next, peering into its surprisingly clear waters and counting jellyfish, then got a pide at a cafe. I caught another cab back to the hotel, and eventually fell asleep, a victim of jet lag…

Transect debrief 7: Brittle-ductile deformation

On the transect trip, I also saw some nice meso-scale “minor” structures that probably formed during Alleghanian deformation. Prominent among the ones that really impressed me were these en echelon tension gash arrays, deforming the Antietam Formation quartz sandstone and well exposed in blocks used to construct the wall along Skyline Drive and the Sandy Bottom Overlook in Shenandoah National Park:

Good Lord! Ain’t those things beautiful? They also give us a lovely sense of the kinematics (relative motions) of the blocks of Antietam sandstone on either side of this sheared zone. In the case of the image above, the left side of the photo has moved “down” relative to the right side. The rock in between has torn and stretched, with the gashes opening up at right angles to the maximum stretching direction. As deformation proceeds, of course, the gashes rotate and deform, folding into “S” shapes.

Here’s one that’s more subtle:

What you’re looking at in the image immediately above is a tension gash array that was a zone of weakness, exploited by later brittle deformation. The fracture which defines the edge of this block cracked through those old brittle-ductile tension gashes and split them clean in half.

Neat, eh? …Now check this out:

Remember the Skolithos trace fossils? Here, you’re looking at a sideways cross section through some cylindrical Skolithos as they are disrupted by this zone of shearing. Note that the burrows tend to be highlighted by rust (hematite) staining: the brown lines that run roughly from the top left of the photo towards the bottom right. But look what happens to the orientation of those tubes where they are cut by the tension gash arrays: they are deflected into a new orientation, rotated from their original orientation!

If that’s a bunch of gobbledygook to you, consider this annotation:

I’ve drawn white lines to show the orientation of the Skolithos tubes in their undeformed and deformed states, colored the tension gashes yellow, and drawn on a set of blue arrows to show my kinematic interpretation (top to the left).

Here’s another block, showing the same phenomenon:

Go ahead. Tell me you’re not impressed with that. I dare you. That is frakking AWESOME.

You are now dismissed.

GSW spring field trip

A few photos from last May’s spring field trip with the Geological Society of Washington… Here’s the group at Chain Bridge Flats (far westernmost-Washington, D.C.), looked at the metamorphic rocks there — a metagraywacke melange  known as the Sykesville Formation.

Another group shot, with field trip leaders Tony (khaki shirt) and Gary (red jacket) Fleming in the foreground:

Euhedral metamorphic pyrite crystals (porphyroblasts):

An elusive bedding plane in the Sykesville Formation (a rare thing to see, as the rock has been pervasively metamorphosed and deformed):

Annotated version of the same, highlighting the grain size change that defines the bedding plane:

Boulder of Cambrian-aged Antietam Formation quartzite, washed ~25 miles downstream by the Potomac River, bearing characteristic Skolithos trace fossils.

A close-up of the side of this boulder, showing another trace fossil, Diplocraterion, as well as one of the Skolithos tubes.

Annotated version of the same photograph:

Finally, another piece of the Antietam Formation, this one only cobble-sized, showing another example of Diplocraterion:

GSW field trips are free and open to the general public. If you’re in the D.C. area, watch the D.C. Geology Events website for opportunities like this, and then come on along and join the fun!