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Clarksburg Shipwreck Q &A with the
Crew
(Some questions have been combined)
Meagan,
Zakia and Jack ask:
How can you
find your way around in dark water? How do you navigate?
We use the same tools people do on the
surface, with just a small twist. We use waterproof flashlights to help
us see and we use guides. In the case of Clarksburg we used a bright red
rope that was marked every 10 ft with a neon pink piece of surveyor
tape. The pink markers were numbered from 10ft to 200ft and stretched
from the most northern ship feature we found to the southernmost
shipwreck fragment. Red is the last color you can see in fading light so
it was visible even in the limited visibility and formed a guideline
around the wreck site. We could easily move up and down the wreck
following the guideline and if we became disoriented underwater all we
had to do was look at the numbers on the line to figure out where we
were on the site.
In other instances
when we don’t have a guideline we use waterproof compasses to plot a
course underwater and find our way back.
Jack and Zakia ask:
How do you put the data together at the
end to make the map? What equipment and methods do you use to
measure distance?
We have a tool kit
that consists of pencils, clipboards, seamstress measuring tapes, reel,
measuring tapes and sheets of frosted Mylar. We can’t use paper because
it would disintegrate underwater, so we use frosted Mylar to write on.
We tape the Mylar to clipboards so it won’t float away and often tie our
pencils to the clipboard, too. Before we go into the water we write
down what we plan to do because underwater it is very easy to forget.
Once underwater we go to work using our measuring tapes to record
dimensions. For detailed work we make quick sketches and set up
dimension matrices recording the facts on the Mylar. When we come out
of the water we transcribe all of our notes onto paper and into our
notebooks in set scientific fashion.
For large
measurements over long distances we use several different measuring
systems, angle-distance, trilateration, triangulation and a three
dimensional mapping system called the phantom pyramid. To do
this we set up one or more datum points on the archaeological site and
use the big reel, measuring tapes to stretch across longer distances.
All of the
measurements and sketches are then assembled back in the lab in a scaled
drawing that ties all the details together. The Clarksburg map will be
to the scale of 1ft = 0.25inches. The shipwreck site is currently 120
ft long by 20 ft wide. So how large will the
scale map be?
Jack asks:
How do we tell which part is what while
mapping?
We don’t always know
what we are mapping so we take the route of mapping every detail.
Knowing ship parts takes study and experience of translating clean,
line-drawings into the imperfect huge structure you see underwater.
Often it is a process of elimination. We often show our drawings and
photographs to other specialists to help identify particularly pesky
features we can’t identify.
Nikki asks:
What type of nails were used for the construction
of this ship?
We found wooden
treenails (pronounced "trunnels"), iron spikes, iron and bronze drift
pins with washers on either-end, and square, large drift spikes that ran
horizontally nailing frame parts together. We also found small, copper
tacks holding the copper sheathing on the outside of the hull and larger
bronze tacks that look like modern roofing nails holding on the
protective lead sheathing along the stempost. We picked up loose
examples of the different styles of fasteners to have them tested for
metallic composition.
Dan asks:
Why can't the ship just be pulled up using ropes?
Usually
by the time we are called into lift a shipwreck it is very old and
fragile. Wood is cellular in structure and over time underwater becomes
more and more fragile – softer and softer. Thus, when lifted we need to
use broad straps or webbing to lift spreading the point of contact over
a larger area, protecting the fragile remains of the hull.
Dustin asks:
How did they build a levee on top of a shipwreck?
It was not done
purposefully. However, neither was the area surveyed for possible
shipwrecks -- and so, the Clarksburg ship was caught in the crack of not
being seen and not being looked for. In truth the levee only just
recently covered the shipwreck with levee rock, known as rip-rap. When
first located a few years ago the ship was completely intact and
standing erect on the bottom. This happened just before the dramatic
effort along the banks of the Sacramento River to reinforce and improve
the levee system. These projects were offered emergency status and thus
did not have to follow the normal laws governing historic preservation.
No surveys were conducted prior to dumping the tons of rock along the
levee. It appears that when the rock was dumped on the shipwreck it
weakened it further and the portside, which is the side we were able to
see and study, broke away and fell over. It now lies side down in the
silt of the lower embankment.
Kevin asks:
Does the water temperature fluctuate with the time
of day?
Yes, but very
little. The air temperature fluctuates more, as does the current. The
water temperature changes more gradually over the seasons, but stays
within the range between about 55˚F and 70˚F. The temperature for the
week we worked was consistently around 64˚F. The high point of air
temperature was around 79˚F. But the current changed over the course of
the day running around 1 to 1.5 knots in the morning and dropping to
around 0.5 knots in the afternoon. This is a "Catch-22" since it is
easier to swim in the lesser current but the disturbed silt clears away
faster in the higher current.
Alex asks:
Does the water temperature affect the preservation
of the ship?
Does the type of wood that the ship was made of affect preservation?
The water temperature
in shallow water sites is not as important as the presence of oxygen.
When oxygen is present then organisms that consume organic materials,
like wood – thrive. When oxygen is not present the wood and other
organics are better preserved. Water cannot drop in temperature below
around 30˚F otherwise it becomes ice. The second important fact to know
about the ship is the type of wood it is constructed from. Harder woods
survive longer while softer woods deteriorate rather rapidly. Ships are
usually built from a number of types of wood. We have collected a
fragment of the keel, a fragment of a plank, a fragment of a frame and a
fragment of wooden nail (treenail) to see if we can define what woods
this ship was built from. Think about speciation. Not all trees grow
everywhere, so what do you think we will be
able to do once we have the types of wood used in the Clarksburg ship?
Sarah asks:
How are we going to know where to turn?
How are we going to know if we went too far?
I am assuming you
meant underwater. That is why we put up the guideline with markers all
along it the very first day on the underwater site in the river. In
clear water it is not so important although on all archaeological sites
there is at least a datum point and often a baseline, whether or not you
can see it. The datum point is tied back into a known location on the
earth’s surface and every measurement on the archaeological site is
related to the datum. In that way the measurements mean something in
the real world and can be put back in place.
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