THE BOB CAMPBELL GEOLOGY MUSEUM

Leather Back Turtle Dig

Museum Curator, Dave Cicimurri participated in a Leather Back Turtle Dig in Summerville SC

	Making the coffer dam The canal was nearly bone dry the day before we began excavating the fossil. Unfortunately, three inches of rain fell that night, and when we arrived at the site, the canal was full of water and the fossil was submerged. The first thing we needed to do before excavation was keep the water away from the shell. We filled canvas bags full of clay from the banks and used these to make a coffer dam.
	Finishing the dam and bailing water We made a ring of sand bags around the fossil, but water kept coming in from between the bags. Clay was used to shore up the outside of the dam, and once we stopped the water we began bailing out the the basin.
	Newly exposed carapace After all of the water was removed, we could see that much of the carapace had already been exposed by its discoverer. Our next task was to determine just how big the shell was, so we used small picks and brushes to gently remove matrix (the surrounding rock) and expose the shell until we reached its perimeter.
	Images of trenching Once we knew how big the shell was, we started to isolate the fossil from the rest of the matrix. Using shovels and picks, we started trenching around the fossil. We needed to be careful because there may have been bones hidden in the matrix, so before digging we probed the area with long awls to find other fossils. After about every three inches into the trench, we probed again.
	Isolated shell In order to have enough room for making the plaster jacket, we made the trench at least a foot wide. We also dug the trench to about 10 inches below the shell.
	Views of shell with compass Before we encased the shell in a plaster jacket, we noted the direction the shell was facing using a compass. In this case, the front of the shell was facing almost due north. From the orientation of the shell we can tell that, 29 million years ago, bottom currents were flowing in the direction of present-day north. Looking at the shell head-on (anterior), we can see that there is a low ridge running down the middle of the shell. There is also a second ridge on each side of the middle ridge, and these curve inward towards the back of the shell. So, the front of the shell is wider than the back of the shell. We can tell which direction the current was flowing because objects that taper at one end will orient themselves so that the wider end is facing the direction the current is moving. Also, a piece of the back end of the shell had become detached and transported a short 12 inches to end up on top of the rest of the shell. Individual bones from the margin of the shell had become detached and isolated.
	Close-up of shell We cleaned the shell up a bit to take some photographs, and in this view you can see the mosaic appearance of the shell. Unlike other sea turtles (like loggerheads and hawksbills), a leatherback carapace is composed of thousands of polygonal plates that go together (articulate) to form something of a flower pattern. Can you see the patterns in this photograph?
	Applying paper to the shell Now that we were ready to make a plaster jacket, we first needed to cover the shell with a separator. In this case we used newspaper, but paper towels and toilet tissue (many more uses for a paleontologist than just the obvious) will work just fine. If we didn’t use a separator, the plaster would stick to the fossil, making it difficult (if not impossible) to remove without damaging the bone.
	Showing the burlap wraps Once all of the exposed bone was covered with newspaper, we made preparations to apply the plaster. To strengthen the plaster jacket, we include strips of burlap. We cut up old coffee bean bags and potato sacks into eight inch strips and rolled them up. The strips were soaked in water to clean out the dirt and oil, and this also helps the plaster adhere. After soaking for a while, the excess water was wrung out.
	Applying the plaster wrap Mixing plaster is an art – too little plaster mixed with water and it will be too thin; too much plaster and the mixture will be too thick and set up too quickly. Temperature can also affect the curing time of the plaster…hot water or air will make the plaster set quicker. Once the plaster was mixed to the desired consistency, we unrolled the burlap strips and then roll them up again in the plaster. This ensured that both sides of the entire strip were coated with plaster. Next, we took one end of the strip and unrolled it onto the specimen. In this case, we began by wrapping the perimeter of the shell.
	We zig-zagged the strips from side to side, then applied another layer from front to back, perpendicular to the first one. We also wrapped bandages across the undercut section of the pedestal; this would help keep matrix from falling out once we flipped the jacket over. All of the leftover plaster in the bucket was spread over the bandages to strengthen the jacket.
	As the plaster was setting, we began undercutting the block. We needed to break the fossil free from its matrix pedestal in order to flip the jacket over. It turned out that this was one of the hardest parts of the excavation. After the block was sufficiently undercut, we wrapped a rope around the jacket. One or two people pulled on the rope at the same time four of us grabbed the margin of the jacket and lifted.
	We successfully flipped the jacket and began removing some excess matrix. This lightened the load and made it easier to maneuver the jacket. We decided not to plaster this side of the jacket because the matrix was so blocky and strong.
	We got permission from the city of Summerville to drive a truck into the canal, and a local resident was kind enough to lend us his trailer. We hitched up the trailer, made a ramp out of loose boards, and manhandled the jacket onto the trailer. After four days of hard work, we were about ready to go.
	After securely lashing the jacket to the trailer, we drove out of the canal. Because the jacket was so heavy, it took a few tries to get the truck up a short hill and back onto the trail. We covered the jacket with a tarp before heading away for good. The fossil is currently being prepared at the SC State Museum, and a leading authority on fossil leatherbacks, Dr. Roger Wood, is eagerly awaiting the chance to study it.
	We stopped for a quick photograph on day two. I’m in the back row on the far left, followed by Jim Knight, Vance McCollum, Mark Bunce, and Paul Bailey. Jim is the Chief Curator of Natural History at the SC State Museum in Columbia. Vance, Mark, and Paul are members of the Lowcountry Fossil Club. Paul discovered the fossil, and the club donated it to the State Museum. Some of the future paleontologists that helped with the excavation (they live in a subdivision adjacent to the canal) are in the front row.