Ballast of the Past

While strolling through the beautiful and historic city of Savannah, Georgia last week, I made sure to pay attention to the thousands of time machines below my feet. Yes, I know, everyone other than geologists stubbornly refer to these objects as “rocks.” Fortunately, though, we earth scientists don’t have to limit our imaginations by using such simplistic labels. These pieces of a pre-human past all have stories to tell of their origin, and sometimes they even connect to our treatment of one another as human beings.

Ballast-Stones-Street-Wall-SavannahA street on the north edge of Savannah, Georgia leading down to the Savannah River, composed of rocks from afar. How did these rocks get there, and what stories do they tell us about themselves and us? (Photo by Anthony Martin.)

Temporal considerations aside, the rocks of Savannah don’t really belong there. This is especially true for those on the north end of town cobbling the roads and reinforcing walls next to the Savannah River. A quick glance at these stones by the geologically informed reveals how these are all foreign to this part of Georgia. Sure enough, most are from across the Atlantic Ocean, with the majority probably originating in the British Isles. Yet they also have been part of Savannah history for at least a few hundred years. What are they, how did they get there, and why are they there?

Studying-Ballast-Stones-SavannahA fine example of how rocks and a geologist (me, in this instance) get along just fine, especially when that geologist kneels in their presence. Note also the stone walls on either side of the street, which also figure into the origin story of these stones. (Photo by Ruth Schowalter.)

These are ballast stones, which filled the holds of ships during the 18th and 19th centuries as they sailed across the Atlantic Ocean from England. Were these ships exporting rocks to eager colonists who wished to collect nostalgic (and solid) reminders of their former homelands? No, ballast stones were used to keep ships weighted down, which helped to stabilize them as they moved across seas both calm and rough.

Once a ship reached Savannah – which began as a British settlement in 1733 – its crew would dump its rocky cargo and replace its relatively uneconomic value with goods grown in Georgia, such as rice, cotton, and indigo. Those economic commodities then went across the ocean, where they were used for food (rice) or textiles (cotton and indigo). Meanwhile, the ballast stones were repurposed as durable materials for the streets, walls, and houses along the Savannah River, as well as in some of the older homes in the historic district of Savannah.

The rocks on the streets and in the walls of Savannah are amazingly varied, reflecting the geological diversity of the United Kingdom and perhaps other places. (Admittedly, I haven’t done an exhaustive literature search on this topic yet: This is only a blog post, y’all.) Igneous, metamorphic, and sedimentary rocks are all represented, but perhaps the most common type I saw was basalt, which is a black, fine-grained extrusive (volcanic) igneous rock.

Ballast-Stones-Savannah-Close-UpA nice sample of the geologically diverse rocks composing a street in downtown Savannah, Georgia. Geologists glancing at this photo will no doubt spot representatives of the Holy Trinity of Lithology in this assemblage: Igneous, Metamorphic, and Sedimentary. Amen! (Scale = size 8 1/2 shoe (mens); photo by Anthony Martin.)

Ballast-Stone-Basalt-SavannahA good example of vesicular basalt, an igneous extrusive (volcanic) rock that formed from hot magma that cooled at or near the surface of the earth, and nowhere near present-day Savannah. The “vesicular” part of its name is from vesicles formed by gases in the magma, evidenced by those little holes in the rock. (Photo by Anthony Martin.)

However, I also saw intrusive (plutonic) igneous rocks, at least one of which was intruded by basalt, defined by a clean, black band cutting across the older rock. Sedimentary rocks included sandstones, some of which were placed parallel to their original bedding, fitting like bricks in some of the walls above the street.

Ballast-Stone-Basalt-Crosscutting-Intrusive-SavannahForget paper and scissors: This time, rock cuts rock. The black band is a basalt dike, which is cutting across the coarser-grained igneous rock, which may be a pegmatitic granite. Based on the simple principle of cross-cutting relations, the basalt is geologically younger than the pegmatite. (Photo by Anthony Martin.)

Ballast-Stone-Sandstone-SavannahAs a sedimentary geologist, I’m always happy to see a sedimentary rock, and this one was no exception. This sandstone had some low-angle cross-bedding, which was likely made by the sorting of sand, moved and deposited by water millions of years ago. (Photo by Anthony Martin.)

At least a few sedimentary rocks even contained fossils, such as a limestone with gorgeous length-wise and cross-sections of crinoid stems. This one was probably from the Carboniferous Period, from more than 300 million years ago. It was next to another limestone containing what looked to me like cyanobacterial or algal structures, called oncolites. Such rocks were common earlier in the Paleozoic Era, say, 450-500 million years ago.

Ballast-Stones-LimestonesLimestones from another land, but now paving a street in Savannah, Georgia. The one on the left bears what I think are algal structures called oncolites, and the one on the right has nicely preserved crinoid parts. Where are they from, and what are their geological ages? I can only answer “Great Britain” for the former, and “Paleozoic” for the latter. But I suspect the oncolititic limestone is older (Cambrian) than the crinoidal limestone (Carboniferous). At any rate, these rocks are not from the Savannah area, which is composed of sands and muds from much more recent rivers and tides.

Ballast-Crinoidal-LimestoneA close-up of that crinoidal limestone, with the length-wise section of a crinoid stem (center bottom) and cross-sections of their columnals throughout. (Photo by Anthony Martin.)

So like most normal people, you are probably wondering how these ballast stones relate to ichnology. For instance, do any of the sedimentary rocks contain trace fossils? Maybe, although I didn’t see any really convincing ones. Only one rock of the many I examined had some possible vertical burrows, exposed as holes in a sandstone cobble.

Ballast-Stone-Trace-Fossils-SavannahA sandstone with some good candidates for trace fossils, in which the holes may be cross-sections of vertical burrows. It may even have a U-shaped burrow, which looks like a little dumbbell when viewed from above (upper right). Sadly, out of all the rocks I saw on the street, I didn’t see any others like this, so I wasn’t able to test my hypothesis any further. (Photo by Anthony Martin.)

But there is another trace here, one much larger and more conceptual than what can be discovered in a single stone. Think of how these ballast stones collectively represent a human trace, tangible evidence of a grand transference of geological heritage from one continent to another.

From more of a moral perspective, however, these ballast stones are also a trace of slavery. The labor of enslaved people – abducted from their homes in western Africa and, like ballast stones, packed into cargo holds on ships and taken to a foreign land – produced the agricultural goods that went back in ships to Europe.

Although slavery was at first banned from Savannah, it was allowed soon after its founding (starting in 1750) and continued after American independence in the latter part of the 18th century. Savannah one of the most productive ports in the world for the shipping of rice and cotton during the antebellum times in the 19th century, and the heinous exploitation of human lives continuing until the advent of the American Civil War in the mid-1860s. This meant more ships arriving over the years, still bringing their ballast stones, and taking back cotton, rice, and other fruits of this cruel labor. Meanwhile, slave labor was also used to construct many of the streets, walls, and homes in Savannah composed of ballast stones.

Ballast-Stones-Street-Walls-Savannah-2A Savannah street and walls, built with rocks from another land, and by people from another land, some of whom did not have a choice in building them.

So there would be far fewer ballast stones on the streets and in the walls of Savannah if not for this brutal part of English and American history. The legacy of these stones also links to the family lineages of millions of African Americans, whether they live in Savannah, other parts of Georgia, the U.S., or abroad. As we walk on these rocks in the streets of Savannah, I am mindful of how their physical weight later became an emotional one, one still carried by many of us as we view and walk on these traces of that past.

IMG_2738African American Family Monument, a bronze sculpture designed by Dorothy Spradley, on River Street in Savannah, Georgia. The foundation – which I think is  composed of more geographically appropriate granite from Elberton, Georgia – is inscribed with the following words by Maya Angelou (1928-2014), which, like the ballast stones, remind us of a past we might like to forget, but should not.

We were stolen, sold and bought together from the African continent. We got on the slave ships together. We lay back to belly in the holds of the slave ships in each others excrement and urine together, sometimes died together, and our lifeless bodies thrown overboard together. Today, we are standing up together, with faith and even some joy.

(For a bit more information about Savannah’s ballast stones, and to see them for yourself while visiting Savannah – which I highly recommend – visit the Historical Markers Database site at Savannah’s Cobblestones.)

Recent Signs of Life

Following a flurry of dozen posts in December 2013 loosely inspired by The 12 Days of Christmas, this site has been morbidly silent, a veritable vacuum of verbosity. This was probably a good thing, as my life has been occupied by a few other tasks and events, and will be in the near future. So in the proud tradition of Buzzfeed and other Web sites that rely on enumerated click bait for their traffic, here are the top five reasons why I haven’t been blogging lately.

5. I finished writing a book. Titled Dinosaurs Without Bones: Dinosaur Lives Revealed by Their Trace Fossils (Pegasus Books), I’ve been working on it since the summer of 2012, and I’m now holding it in my hands, which is a sure sign that it’s done. Overall, I’m very pleased with how it came out, and even more pleased that it’s out and available for others to enjoy reading. What’s it about? Just re-read the title, but if you’re still not quite sure, I guess you’ll have to get the book and read the whole thing.

Dinosaurs-Without-Bones-BookHere’s a trace of what I’ve been thinking and writing since 2012. Hope you like it. (Photograph by Anthony Martin, taken at home.)

4. I taught a field course in the Bahamas. Once every two years, I organize and teach a field course for Environmental Sciences students at my university. This course, which lasts about 10 days, is held during winter break at the Gerace Research Centre on San Salvador Island, Bahamas. Yes, I know, you’re thinking the following: “Can I go?” “Do you need a field assistant?” “That must be nice!” “Poor baby!” [the last of these said sarcastically]. No offense, but I don’t think you would last a day in this course. (Yes, you. And especially you.) It’s a physically demanding course, with land- and water-based field work every day, along with nighttime lectures and discussions, and that happens all before everyone walks 3 km to the nearest bar. Nonetheless, it’s a wildly successful course, in which my initially scared-of-the-outdoors-and-anything-alive-or-dead students are transformed into something approaching field-hardy scientists by the end of their time on the island. I’ll write separately about our latest experiences in an upcoming post, so be looking for that.

Outdoor-Classroom-BahamasOnce every two years, this is my classroom. Notice how I even got a student to teach everyone else that day. Not seen in this photo? Fruity drinks with paper umbrellas. (Photograph by Anthony Martin, taken on San Salvador Island, Bahamas.)

3. I gave my first public talk about the new book. First, let me heap some praise on one of the most awesome organizations in the Atlanta area, the Atlanta Science Tavern. With nearly 4,000 members, the Atlanta Science Tavern organizes several science-related talks and events held each month, they’re involved in the upcoming Atlanta Science Festival, and in charge of the Science Track at the Decatur Book Festival, all of which I reckon makes them a force to be, well, reckoned with. Anyway, they invited me to speak about my book as part of their annual Darwin Day Dinner event last month, and I happily complied. It was also great fun being the warm-up act for paleobotantist (and friend) Dr. Melanie DeVore, who spoke later that evening taught us about Darwin’s connection to the “abominable mystery” surrounding the evolution of flowering plants.

2. I co-wrote and submitted a research paper. Sometimes when I’m on San Salvador Island and teaching my students in the field, we make discoveries. So when someone tells you matter-of-factly that teaching and research rarely converge, this pronouncement can be, like, totally falsified when you’re teaching outside of a classroom. Which is to say, outside.

For example, on December 30, while with my students at an coastal limestone outcrop on San Salvador, my student teaching assistant (let’s call her “Meredith”) pointed to some features and said, “Hey Dr. Martin, are these ___________?” To which I replied, “Why yes, I think those are ______________!” (I’d be glad to tell you what they are, but first they have to go through peer review.) So in the past few months, I wrote a draft of a short research paper reporting the find, “Meredith” added her editing suggestions, and we submitted it to an open-access journal for possible publication. But what was really neat about this discovery was that we shared it with the other students in the field, right then, right there, and used it as a teaching lesson on what you should do when making a potential fossil discovery in the field. Take that, false dilemma!

Wave-Innundation-San-SalavdorWhat is life, but a coastal limestone outcrop with a paleosol that is daily immersed by tides and inundated by waves, awaiting discovery of its hidden trace fossils, which are revealed by those same tides and waves? And by the way, watch out for those waves. (Photograph by “Meredith” – which may or may not be her real name – and taken on San Salvador Island, Bahamas.)

1. I’ve been teaching (more so). No matter how much it might pain pundits who love to bash those unproductive academics for their non-existent class loads, cushy tenure, exorbitant pay, and – most galling of all – academic freedom, I’ve been teaching 40+ students this semester in two classes, advising a senior honors-thesis student, and helping other students pick out courses for study-abroad programs. Incidentally, I’m also not tenured, my pay is far lower than that of the aforementioned pundits make, and I have no academic freedom (see previous statement about lack of tenure).

For a good summary list of what professors actually do in their jobs, read this. But if you’re one of those people who won’t have your mind changed by evidence-based reasoning, then by all means go back to watching your favorite cable-news show and watch people shout at one another about how climate change is a hoax, whether or not mermaids and Megalodon (or, better yet, a mermaid-eating Megalodon) really do exist, and other fascinating fare. Regardless, I’ve had fun teaching these two classes, although I’m guilty of putting off grading the Bahamas field-course reports. Que sera, sera.

0.5. (Bet you thought you were done, didn’t you?) I attended two conferences in the past few weeks. The first conference is one that meets only once every five years, the North American Paleontological Convention. It’s normally a wonderful conference, and this one – held in Gainesville, Florida and hosted by attended by about 500 paleontologists of all types – was no exception. Indeed, it’s one of the few times we can get micropaleontologists, paleobotantists, invertebrate paleontologists, vertebrate paleontologists, taphonomists, and even ichnologists under the same roof. Other than learning heaps from my paleontological ilk, I presented a talk summarizing Cretaceous trace fossil research I’ve done with colleagues in Victoria, Australia since 2006, and other colleagues of mine at the North Carolina Museum of Natural Sciences and I co-authored a poster about Ediacaran fossils in North Carolina.Paleontologist-Barbie-NAPCHere’s that poster on Ediacaran fossils from North Carolina, coauthored with Patricia Weaver and Chris Tacker from the North Carolina Museum of Natural Sciences. Notice it’s also undergoing peer-review by our hero, Paleontologist Barbie. Is there nothing she can’t do? (Photograph by Anthony Martin, taken in Gainesville, Florida.)

The other conference, still fresh on my mind, was Science Online 2014, which was held in Raleigh, North Carolina last week. It was my first time to this conference, and my main reason for going was to promote my book, which was on display there and being given away to lucky attendees in a raffle. But I also had the nice fringe benefits of meeting many very nice (and very smart) folks from the science-communication community who I had only known previously through digital media, while also learning much about online-science communication during sessions on a variety of topics. From what I gathered, a good time was had by most.

What’s coming up in the next future? Plenty! For one, Dinosaurs Without Bones is officially released this Thursday, May 6, 2014. So I’ll probably have something to say about that. Ta-ta for now, and thanks for reading about my latest signs of life, which may or may not preserve in the fossil record.

Ghost Shrimp Whisperer

When you hear the word “shrimp,” you probably picture those that show up in grocery stores and restaurants throughout the world, which are then consumed voraciously by their terrestrial admirers. Also, some recent attention has been given to mantis shrimp, and deservedly so, because they are among the most gorgeous and terrifying of marine invertebrates today. But there are other marine crustaceans bearing the name “shrimp” that are neither gracing seafood buffets nor awesome predators, yet are worthy of our adoration, documentary films, and epic songs, the latter of which will be no doubt performed on Eurovision 2014. Yes, you guessed it: I’m talking about ghost shrimp.

Ghost-Shrimp-Burrow-Tracks-JekyllWhat’s this? We’re looking down on the surface of a Georgia beach at low tide. The collapsed top of a ghost shrimp burrow is in the lower left, but it’s connected to a trackway, which ends in a shallow horizontal burrow, which holds the maker of all three types of traces. Lots of other ghost-shrimp burrow tops are in the upper part of the photo, too. Life doesn’t get much better than this for an ichnologist. You may now envy me. (Photo by Anthony Martin, taken on Jekyll Island, Georgia; scale in centimeters. )

Why ghost shrimp? Because they can burrow like nobody’s business. Take a typical ghost shrimp in the Bahamas or the Caribbean, such as Glypterus acanthochirus. This crustacean is only about 10-cm (4-in) long, but if it lives for eight years and burrows continuously through that time, it will have processed a cubic meter of sediment. Individual ghost-shrimp burrows can go as deep as 5 m (16 ft). These would be like a human shoveling more than a cubic kilometer of dirt, or a vertical shaft about 100 m (330 ft) deep, but without a shovel, backhoes, augers, drilling rigs, or other tools. These vertical shafts then connect with extensive branching tunnels, making complicated networks in the sand and mud below the level of the low tide. Now multiply that industriousness by millions, and we’re talking about enormous volumes of sediment processed by ghost shrimp in their respective shallow-water environments. Ghost shrimp are like the ants of the ocean, only not as organized: no queens, workers, soldiers, or other divisions of labor, just lots of individual shrimp burrowing, eating, mating, and defecating.

Ghost-Shrimp-Burrow-TopsEvery one of these holes is the top of an occupied ghost-shrimp burrow. Now imagine meters-long vertical shafts from each of these going down into the beach sand, then turning into branching horizontal networks of such grandeur, they would further embarrass naked moles rats, which are already apologizing for how they look. (Photo by Anthony Martin, taken on Sapelo Island, Georgia. Human foot (upper right), still attached to human, for scale.)

Ghost shrimp share a common ancestor with crabs, lobsters, crayfish, and shrimp, all of these having four pairs of walking legs and one pair of claws. (Mantis shrimp are actually not true shrimp – or even decapods – but stomatopods.) Ghost shrimp are also known by marine biologists and ichnologists as callianassid shrimp, belonging to an evolutionarily linked group (clade), Callianassidae.They burrow through sand and mud using their front two claws, but also carry sediment on their other legs. Ghost shrimp are also well-known for depositing much of the mud on Georgia beaches as elegantly packaged little cylindrical fecal pellets. These bear enough of a resemblance to “chocolate sprinkles” on cupcakes that they become tempting to sample, until you remember that they’re, like, you know, fecal.

Ghost-Shrimp-Fecal-PelletsGhost-shrimp fecal pellets, each about 5 mm long, and recently ejected by a ghost shrimp through the top of the burrow, which is the little hole just to the right. If you use them with any cupcake recipes, let me know how that worked for you. (Photo taken by Anthony Martin on St. Catherines Island, Georgia.)

Geologists love ghost shrimp, too, because of how their burrows are so numerous, fossilize easily, and are sensitive shoreline indicators. I wrote about this before with regard to how geologists in the 1960s were able to map ancient barrier islands of the Georgia coastal plain by looking for trace fossils of these burrows. Since then, geologists and paleontologists have identified and applied these sorts of trace fossils worldwide, and in rocks from the Permian Period to the Pleistocene Epoch.

I could prattle on about ghost shrimp and their ichnological incredibleness for the rest of the year, but will spare you of that, gentle reader, and instead will get to the point of this post. Just when I thought I’d learned nearly everything I needed to know about ghost-shrimp ichnology, one shrimp decided I needed to have my eyes opened to some traces I had never seen them make before just a few months ago. I mentioned these traces briefly in a previous blog post, when I was teaching undergraduate students from my barrier-islands class on Jekyll Island (Georgia) in mid-March. They were tracks and a shallow horizontal burrow made on the surface on the northernmost beach of Jekyll Island, and they were made by a ghost shrimp. How do I know they were made by a ghost shrimp? Well, maybe because they had a ghost shrimp attached to them, but that’s beside the point.

Ghost-Shrimp-Tracks-Burrow-Left-JekyllA close-up of the left side of the trackway shows more clearly how it definitely is connected to the funneled top of a burrow. The trackway shows small pointed impressions and a central groove in places, showing that this is an animal with legs and a tail, respectively. The irregular path of the trackway is a record of pauses, where the trackmaker stopped briefly before moving on. The body length of the tracemaker is subtly revealed along the way too, but explaining that would require a more advanced lesson in ichnology. So maybe another time. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

Ghost-Shrimp-Tracks-Burrow-Right-JekyllThe right side of the trackway, ending in a short and shallow horizontal tunnel, just under the sandy beach surface. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

Ghost-Shrimp-Tracks-Burrow-Closeup-JekyllThe trackway and tail-trail ends in a tunnel with a thin roof of sand. The bilobed pattern was made by the claws and other legs on either side moving sand up and around the body of the tracemaker. Notice the roof collapsed a little on the right, and that its tail is sticking out on the left: kind of like hiding under a too-short blanket. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

Ghost-Shrimp-JekyllTa-da – the tracemaker revealed! I’m fairly sure this is a Georgia ghost shrimp (Biffarius biformis), but would appreciate all of those marine biologists out there to correct me if I’m wrong. (And not those fake marine biologists, either.) Rest assured, after showing it to my students and allowing them to photograph it, I put it back in the ocean, where it burrowed happily ever after. Unless it died, that is. (Photo by Anthony Martin, taken on Jekyll Island, Georgia.)

What truly amazed me about these traces, though, was their rarity. As I shared with my students, in more than 15 years of field work on the Georgia coast, I had never seen anything like this sequence of traces. Even better, the tracemaker was right there, and like the period at the end of a sentence in the story.

Furthermore, the story told by these traces was that something must have threatened the life of this shrimp to cause it to behave in such an unusual way. These shrimp almost never see the light of day, and prefer to stay deep in their burrows, away from the prying eyes and beaks of shorebirds, fish, and other predators. Consequently, they remain largely invisible to humans; hence the “ghost” part of their nickname. This means something very bad must have happened to this one in its burrow, prompting it to abandon its refuge and expose itself so vulnerably. It would be like a fire forcing people out of their fortified underground bunkers, but when they know tyrannosaurs are lurking just outside. Damned if you do, damned if you don’t, but something in this ghost shrimp’s evolutionary program made it take the path of the lesser damned.

What happened? Did a predator find its way into the burrow and chase it out? Was it a chemical cue of some sort, like oxygen-poor water flooding into the bottom of its burrow? Was it competition from another ghost shrimp, evicting it from its home? Was it a mate that decided it had enough of sharing this burrow and needed some “alone time,” or took up with another more comely shrimp? I don’t know, but it made for a good little mystery, yet another posed by life traces on a Georgia beach, and one I was delighted to discover and share with my students on Jekyll Island.