Sunday, September 6, 2009
Stratigraphy of the Seven Site
Nikumaroro is an atoll about 4.5 miles long and a 1.5 mile wide, with a lagoon in the middle. The windward (NE) side of the atoll is relatively narrow, with a topography dominated by a more or less central surge ridge paralleling the ocean and lagoon shores. In other words, constant battering by the sea has gradually built up a ridge of broken coral parallel to the shore; it’s no more than 10 meters high, and usually a good deal lower. The Seven Site lies along the crest and SW (lagoon-ward) side of the surge ridge.
Because it’s made up of broken coral, the ridge is roughly equivalent to a pile of gravel, though the individual pieces of coral are more variable than in your average gravel pile, ranging from fingernail size to the size of your head. In the one place we’ve dug relatively deeply (about 110 cm., at the site of the hole where we think the cranium might have been buried, near the SW base of the ridge), the coral becomes relatively consolidated at about 1 meter depth into something resembling bedrock, but with extensive void spaces.
The coarse nature of the material making up the surge ridge means that fine material – sand, small pieces of coral, organic detritus, and many artifacts – filter through the topmost stratum and wind up somewhat lower. We’ve found that there’s a relatively stable “surface” at about 10 cm. depth; almost everything we’ve found in the way of features and artifacts have been in that top 10 cm. But not in the top 5 cm. – that’s made up of relatively coarse material through which (or through voids around which) the lighter material has sifted. As a result, a close inspection of the surface itself may reveal very little, but once the top 5 cm. or so is raked or scraped away, one begins to encounter fire features and artifacts – which, however, peter out at about 10 cm. depth.
Typical stratigraphy (North wall of "hole" excavation"