One of the most important, dirty and labor intensive aspects of terrestrial archaeology is screening excavated sediments: as above, so below. Well, sort of. Working underwater makes everything more complicated.
Although archaeologists dig very carefully, they often screen excavated material to make sure no important clues are missed. On land, this is most often accomplished by filling buckets with back dirt and dumping them through a 1/4″ or 1/8″ screen. The archaeologist shakes the screen and the dirt sifts though, leaving artifacts behind.
While the principle is the same for underwater sites, the logistics are nightmarish.
On the Aucilla River Project, we pumped fill directly to the surface, where it was screened on a floating barge (see a description here). This was practical because we were mostly working with fine sediment, and the current mostly carried back dirt away from the site. We rarely clogged the dredge, but when we did, we often had to bring 30 ft. of heavy semi-flexible hose to the surface to fix it – a major undertaking by any account.
Warwick is buried in a matrix of sand, shells and chunks of coral. The dredge clogs constantly. Tiny pieces of branchy coral interlock their arms as they tumble down the hose and quickly form jams. The spinney oyster shells were also perfectly designed by some malicious entity to pivot, catch and block the hose. We use short lengths of semi-flexible hose so we can jam rebar down them to break up blockages. We also make each section easy to disassemble so we can troubleshoot the dredge without bouncing to the surface.
The dredge is easy to clear this way – the tradeoff, however, is that we screen underwater.
While one diver excavates, brushing sediment and coral into the dredge head, a second diver is stationed at the exhaust, which blows into a screen. We engineered our screen by bolting two traditional terrestrial screens together at right angles.
Artifacts, ballast stones, chunks of coral and shells collide with the mesh backboard and settle on the bottom, while the finer silts and sediments blow on through.
The dredge is powerful, and the exhaust will whip around erratically if not tied off to a heavy object, usually a cinder block. When you screen, you are surrounded by a low lying cloud of sediment and the roaring noise of the exhaust.
A diver searches through the spoil, looking for tiny items that were inadvertently sucked up with the dredge. You pick up a handful of spoil, and brush the shells and coral from your hand. They slowly precipitate down through the water column, leaving only artifacts behind. Most often, these are small pieces of pottery which are hard to distinguish from the surrounding matrix. Sometimes we also identify concreted nails, which look identical to coral on first sight.
Fish come in to feed from the nutrient plume. They move hypnotically, darting in and out around you. Sometimes they bite your wrists, or other exposed skin. Aside from nibbles, it is easy to become mesmerized while watching the schools of Breams cycling though the slowly boiling silt cloud.
At times the sediment cloud rises completely about you – creating whiteout conditions. We continue to screen until we can no longer see our hands just in front of our faces. Working blind, we transfer the spoil to mesh bags, pile them in a crate and send them to the surface. The crew then paws quickly through the shells and ballast on the bow of the dive barge.
Keep on looking
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