What is underwater archaeology? What are the techniques and developments in underwater archaeology? Information about underwater achaeology.
By the second half of the 20th century, equipment and techniques had developed to such a point that scientific archaeology could be practiced underwater as well as on land. Underwater archaeology has the same aims and basic methods as archaeology on land; the difference is the environment in which the archaeologist works. As the necessity arose, archaeologists adapted themselves to work in high altitudes, low altitudes, deserts, dense forests, hard soils, soft soils, frozen soils, tropical climates, arctic climates, and any other kinds of environment the lands of this world had to offer. When technological advances made it possible to undertake proper archaeological researches beneath the surface of the water, archaeologists went into this new environment.
With the possible exception of certain kinds of caves, no environment is as different from normal experience as that faced by the archaeologist working underwater. He must have his own supply of air, must be protected against the cold, and must be able to see, move about, and communicate. Underwater currents, even mild ones, cause more problems than the winds on land. Because of this different environment, the archaeologist working underwater has had to employ special methods to achieve the same ends as he would expect to achieve on the land.
Much of what popularly passes for underwater archaeology is actually pot-hunting and looting of sites or at best is a kind of salvage. In general, people who dive for adventure or treasure are not archaeologists and become bored by the detailed and methodical procedures they have to follow when working under the direction of an archaeologist. Moreover, it is virtually impossible to expect an adventurer-diver to attend a university for the eight or more years it would take him to reach the doctoral level of archaeological proficiency. It is much more practical for the archaeologist to learn to dive. He can then supervise underwater excavation and direct and instruct more experienced divers in their archaeological tasks. Theoretically, a nonliving archaeologist could go underwater in a diving bell or similar apparatus and personally supervise the work of divers with whom he would be in direct communication by telephone. But by and large the archaeologists whose interests lead them to underwater sites should learn to use underwater breathing apparatus.
Recovery of a Bronze Age Cargo.
High standards of underwater archaeological work have been achieved by George F. Bass, assistant curator of the Mediterranean Section of the University of Pennsylvania Museum. Bass directed the museum’s underwater excavations at Cape Gelidonya and Yassi Ada in the Mediterranean Sea off the coast of Turkey. The Cape Gelidonya site was among those found in 1958-1959 by Peter Throckmorton, who was experienced in both diving and archaeology. Throckmorton’s method of surveying underwater sites—shipwrecks in this instance—was somewhat parallel to the survey methods of land-based archaeologists. Instead of using farmers, herdsmen, road builders, and the like as informants about possible sites, he used sponge divers. Walking the bottom of the sea in search of sponges, these men obviously had information about what was beneath the water in the areas where they made their living. Throckmorton spent part of a year living on a boat with a group of sponge divers. He dove with them, and questioned them about everything they had seen. He took notes and recorded the locations of ancient shipwrecks.
Although he learned of the Cape Gelidonya site at this time, it was not until the next year and by other means that he was able to reach the site. Then samples of the sunken ship’s cargo were brought to the surface and were studied by experts. They reported that the wreck dated to about 1200 b.c. and belonged to the Late Bronze Age. At this point Throckmorton, on advice of the Council of Underwater Archaeology (an organization with headquarters in San Francisco), asked the University of Pennsylvania Museum for help. The result was an expedition led by archaeologist George F. Bass, who learned to dive at the YMCA. In addition to Bass, the expedition consisted of Peter Throckmorton as technical adviser, nine divers, and an archaeologist in charge of conserving and recording finds.
The expedition camp was situated on a narrow beach a few miles from the site. Each day the personnel were taken to the wreck in local sponge boats. Once there, each of the eight divers was able to work little more than one hour per day on the site, which lay 90 to 95 feet (27 to 29 meters) below the surface of the water. The site consisted of the remains of a wooden ship and its cargo, encased in a rocklike concretion with occasional protrusions of metal objects. The first step was a mapping of the concreted mass. This was done by a number of photographic montages. Positions of visible artifacts were plotted on sheets of frosted plastic. Then massive sections of the concretion-covered cargo were cut by divers, using hammers and chisels. These sections were hoisted by winch and cable to the sponge boat that was used as expedition tender. Then they were taken ashore and reassembled to reproduce the original mass. The positions of the sections could be determined from the photographic montages and from the location of visible artifacts plotted under water.
After the concreted sections had been reassembled on the beach at the expedition camp and had again been photographed, the actual excavation was begun. In effect, the site had been transferred from beneath the water to the land. The concretion was removed from the cargo portion of the site by means of assorted hammers and chisels and an electric vibrating point. The exposed artifacts and other cultural objects were mapped and photographed in place and in exactly the same relationship to one another as when the ship rested on the bottom of the sea. Concreted parts of the ship in which wood was still preserved had been gently raised by means of air-filled plastic balloons. Although very little of the ship remained, the wooden parts thus recovered provided information on the method of ship construction—the first such information for a ship of the Bronze Age.
The results of the Cape Gelidonya expedition were of great importance not only for the new information on ship construction but also for the chance to examine the cargo of copper ingots and bronze scrap that had been packed in wicker baskets. This cargo provided new data on Bronze Age sea trade and the nations participating in it. From the standpoint of underwater archaeology this was the “first methodical excavation carried to completion.”
Recovery of a Byzantine Ship.
In 1961 and 1962 the University of Pennsylvania Museum, assisted by the National Geographic Society, undertook excavation of another wreck site charted by Peter Throckmorton on his underwater survey. At Yassi Ada, under the direction of George Bass, archaeology beneath the water achieved standards equal to those of the best archaeological work above water. The site was the wreck of a Byzantine ship which sank with its cargo in the 7th century a.d. It lay 120 feet ( 36.6 meters) beneath the surface of the Mediterranean off the coast of Turkey. Expedition headquarters were maintained on shore at Bodrum, 16 miles (25.7 kilometers) from the site, and on a flat barge securely anchored just above the wreck. The expedition staff of 15 experts included a classicist, an art historian, architects, draftsmen, photographers, a geologist, a medical doctor, and a mechanic. Though many of the staff members had no diving experience before they began this operation, the personnel of the expedition completed nearly 6,000 working dives in depths ranging between 100 and 150 feet (30.5 and 45.7 meters). This expedition is significant in the history of archaeology because, for the first time, a ship was excavated completely in situ on the bottom of the sea. Moreover, the whole project was done according to standards as exacting as those used on land.
First, the diving archaeologists cleaned the seaweed from the wreck with wire scrubbing brushes so that the site could be mapped and photographed. Then every visible object and significant point was labeled with a numbered plastic tag facing upward and held in position by a stiff wire pin so that tags could be identified in photographs. Next the mapping began. At first, specially constructed plane tables were used, and directions and elevations were recorded on sheets of frosted plastic pinned to the table tops. This method, however, required three divers and at times was hampered by poor visibility due to muddied water. Next a mapping frame was substituted for the plane tables. It consisted of a square of pipes, 5 meters (191.8 inches) on each side, which could be leveled to an absolute horizontal by means of adjustable legs. Bight angles from two sides of the square were obtained by a sliding horizontal beam to which was attached an adjustable vertical pole. Both beam and pole were calibrated in centimeters. When the bottom of the vertical pole was placed on any object, the elevation of that object and its coordinates in the square were obtained.
But finally an even faster and easier system of measuring was devised, a variation of one that is frequently used on land. A system of grids two and three meters (78.7 and 118.1 inches) square was constructed of metal frames and crisscrossing wires. These grids were laid over sections of the wreck, and the sections were drawn to scale by divers using gridded sheets of plastic and special pencils. Horizontal measurements were taken from the small wired squares within the grid, and then elevations were taken at the four corners of the grid frame either by plane table or by means of the large mapping frame, whichever was easier to use at the time.
After the objects had been photographed, mapped, and listed in place, they were removed by the divers, who began with the upper layer of cargo. The ship held 900 amphoras (large earthenware jars). About 100 were taken to the surface, while the others were stored underwater until needed for study or exhibition. A number of the amphoras were raised to the surface by being filled with air so that they rose slowly like balloons and were retrieved by expedition members on the barge.
The divers either used aqualungs or obtained their air supply from “hookah” hoses—so called because they resembled the tubes of Turkish water pipes. Hoses were hitched to air compressors on the barge. Each pair of divers had to decompress for 21 minutes at the end of their dives so that they would not get the bends, a paralyzing or killing condition that would result if they came to the surface too quickly. In order that this necessary decompression time not be wasted, short air hoses were run to the underwater decompression levels or stations beneath the barge. There the ascending divers transferred from their long hookahs to the shorter hoses, and the long hookahs were taken up to be used by descending divers. The divers waiting out their decompression time wrote reports of what they had accomplished on the dig. These reports were written with the aid of suitable clipboards and pencils hung by cords from the barge floating above. When the diver wished to send a written message to the barge, he pulled a string attached to a camel bell on the barge. The clipboard was then pulled up to the barge.
When the excavators had removed one level of cargo, there often remained a bare layer of sand above the next layer of cultural materials. To remove this sand and search for artifacts in it, a device called an airlift was used. The airlift is a kind of large vacuum cleaner with a suction hose made of metal, reinforced rubber, or plastic. Air from a compressor on the surface is pumped into the suction hose near its bottom. As the air rises to the surface it creates suction at the bottom of the tube, and this powerful suction pulls sand, mud, and cultural objects small enough to enter the tube to the surface.
The airlift used in the Bass operation had a tube about 6 inches (15.2 centimeters) in diameter that was securely anchored in place at the site and was held upright by a float. At its top end there was a large wire basket that funneled down into a large cloth bag. When the sand, mud, and cultural materials reached the wire basket, the currents carried away the sand and mud, but the cultural materials and fragments of shell and small stones passed into the cloth bag. When the bag was full, it was hauled to the barge by a rope; then the contents of the bag were handled as if on a land dig. Considerable skill is required in excavating properly with an airlift. Bass found that the best method was to keep the suction end of the tube a few inches away from the sea bottom and gently to sweep the sand and mud toward the tube with the hand. This much care is not necessary when using the airlift to excavate and remove sterile deposits or to cut a trench around a site.
As each layer of cargo and other cultural material was excavated from each section of the Byzantine wreck, these layers were not only mapped and drawn in place but also photographed as well. Underwater cameras were used to take photographs from fixed positions just above the part of the site that was to be recorded photographically. The fixed camera positions were at the tops of metal towers 4 meters (157.5 inches) high, which fitted on top of a rectangular scaffolding of angle iron constructed in 6-meter (236.2-inch) steps over the whole site. The steps were necessary because the wreck lay on a sloping sea bottom. The pipe legs supporting these steps were adjustable, allowing the archaeologists to keep them at fixed distances above the excavations and to keep them level with the aid of a spirit level. The underwater cameras fitted into a slot at the top of each photographic tower, thus assuring precise control of the underwater pictures.
Eventually the excavations reached the remains of the ship’s timbers. By this time every scrap of material, cultural or noncultural, had been mapped and photographed in place, removed to the surface, cleaned, cataloged, described, and conserved. The architect incorporated the grid maps into a master plan containing every exposed portion of the site, both horizontally and vertically, with each individual find accurately plotted in its original position.
In excavating the remains of the ship’s hull, the divers faced an unexpected situation. When uncovered, the wooden fragments drifted away with the current or were displaced by the slightest motion of the divers. To correct this situation Bass obtained about two thousand bicycle spokes and ground one end of each to a sharp point. The steel pins thus produced were pushed through pieces of wood, thereby holding the entire wreck together and to the bottom of the sea until it could be completely uncovered, examined, measured, photographed, and mapped. There was an accurate record of every surviving scrap of wood and every nail hole, bolt hole, score-line, mortise, and angle, as well as of the relationship of parts. The wood from the ship was raised gently by free divers who placed it in a special wire basket 18 feet (5.5 meters) long and walked it upslope to the shore of a nearby island. The raised wood was kept in tubs of water so that it would not shrink or disintegrate, as it would if allowed to dry without chemical treatment. One widely used treatment employs polyethylene glycol, which can be dissolved in water, soaks into wood cells, and replaces missing cell matter with a hardl plasticlike substance as the water dries out.
The 7th century Byzantine ship excavated at Yassi Ada was a merchant vessel 70 feet (18.3 meters) long. The lower part of the hull was constructed in the routine Graeco-Boman manner with planks joined at their edges with tenons and ribs added later, but the upper part of the hull was built in the modern fashion with strakes or planks placed on a skeleton work of frames. The cabin was located toward the stern of the vessel. Inside the cabin on the left side was a hearth of flat tiles resting on a bed of clay supported by iron bars. The cabin itself seems to have had a roof covered by tiles. Inside the cabin were found the personal possessions of the captain and the crew. Just behind the cabin was the ship’s large water jar. Among the furnishings of the ship were pottery, cooking utensils, and oil lamps. There were also steelyards for weighing the cargo and metal tools for cutting firewood and repairing the vessel. The ship carried 11 large iron anchors. The cargo carried by the vessel consisted of 900 amphoras, or large ceramic vessels, which originally contained wine and weighed about 100 pounds each when full. At some time in the 7th century this ship, while engaged in trade on the Mediterranean Sea, was wrecked off the coast of Turkey. There it lay unknown until the mid-20th century.
A brief summary cannot describe all the techniques of underwater archaeology. However, this study should show that undersea investigations can follow the exacting scientific standards that characterize the work of modern archaeologists in any environment.