17 April 2008
Playing catch-up: In which Cookie finally talks about some Boston geology
Posted by Jessica Ball
Now that I’ve put my brain back together after all the grad school insanity, I really should finish up some of the posts I’ve left hanging. It seems like months since I was in Boston, even though it’s only been…well, weeks. Two very long weeks. Anyway, this one’s about the geology I came up with on the last day I was there, walking the “Freedom Trail”. I probably walked about ten miles that day, mostly because I got out early and everything I wanted to see was closed, which meant a lot of backtracking.
I started out in the North End, where I had a lovely time looking at the outside of Christ Church (the Old North Church). Can’t interrupt services, after all, especially when they’re nice enough to open the place to visitors the rest of the day. (This isn’t really geology, unless you talk about the clay that went into the bricks, but it was pretty.)
Just as you cross out of the North End (going over the I-93 tunnel), there’s a nice little park between Cross Street and Surface Road. And in that park, they have lots of paving stone. It’s really pretty, too, and worth the staring when you sit down in the middle of the sidewalk to get a better look at it. (This one’s pretty big, about 3×4 feet.)
Whee, isoclinal folding! Not migmatites, although Callan’s entry made me hope I was lucky enough to have found them in Boston – more likely just some really squished gneisses. I don’t know much about geology in the northeast, but maybe someone can hazard a guess as to where the city got the stone. Here’s a smaller sample with my foot for scale.
For the soft rock crowd, we have the plaza outside Faneuil Hall Marketplace, which claims to mark the original Boston Harbor shoreline. There are a lot of other strange little markings carved into the stones here – maybe someone from the crew of National Treasure got drunk and decided to add a few extra “clues” for the villains to follow.
Just across the street to the north is the New England Holocaust Memorial. It’s really quite a clever use of an industrial setting: there are a series of six hollow glass towers built over a line of steam vents. The steam rises up into the towers (or chimneys?), which are engraved with six million numbers representing those killed in the Shoah. Each tower represents one of the principle death camps.
The memorial was very haunting, but what also interested me were the construction materials. Here’s an example of the stone used throughout:
The memorial website says it’s “black granite”, which I suspect is more a building supply term than anything a geologist would recognize. Maybe some sort of diabase? I didn’t think it would be respectful to be crouching next to it with a hand lens and a pocketknife, so I didn’t look all that closely. If anyone can hazard a guess as to what “black granite” translates to, go for it.
There are two burial grounds on the Freedom Trail, and, unlike places that use lots of marble, Bostonians of the past seem to have favored more robust materials. There are many headstones dating from the 17th century, for instance, and despite Boston probably having a good share of chemical weathering processes to offer, the engravings are still readable. This one is in the King’s Chapel Burying Ground, and it has alternating bands of silty and sandy material – perhaps even a turbidite deposit? There’s a lot of folding (even to the point of being overturned) in the middle, but none at the bottom, which is puzzling.
The Granary Burying Ground has some witty commentary in granite.
Finally, a contribution to the Airliner Chronicles. This is shortly after takeoff, looking down on some mosquito ditches in a salt marsh. Mosquito ditches were dug into marshes in the Colonial period to drain standing water and sometimes to mark property lines. In later times, particularly after the Civil War, when soldiers were bringing home malaria, they were dug to control the mosquito populations. One of my undergrad classmates did her senior thesis on the effects of these ditches on tidal flux, and apparently many areas are removing them entirely, because they eliminate wetlands and affect the volume of fresh water that reaches the salt marshes.
I probably won’t bother with the MT photos, since I won’t be going there (and even the UPers admitted that the place wasn’t very pretty at this time of year). I will be hiking the Billy Goat Trail this weekend with Callan, though, and I’ll definitely have some good photos from that. If he doesn’t beat me to it, that is. 🙂
Any idea what the glass composition is?
Where I am from, the local gabbro is often used and sold as black granite, although it is clearly not granite.
Sadly, there was no obsidian around, so I’d go with SiO2 + Na2CO3 + CaCO3 +/- Pb, Bo, Ba, Fe, Na2SO4, NaCl, etc etc. Run-of-the-mill, boring old clear glass. But by golly, who knows where that silica came from!