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Day 19 (July 14, 2011): Excavation 101

July 14, 2011

Although I’m at Lake Victoria, I’ll be getting nearly daily updates by email from the research team at Olorgesailie. We’re just getting started with our studies on the Homa Peninsula, so for the next couple days, I’ll give you the updates from Olorgesailie.

a man sits in excavating in the floor of an archeological site. the floor is marked by twenty, two meter squares using twine and nails.
The excavation grid is laid out at right angles with twine and nails. One of the crew members digs several spits below the surrounding squares
There’s serious preparation going on to carry out a lift of new fossils and artifacts exposed by the excavators at BOK-2. I have a feeling this site will be incredibly productive this year. So Katie wanted to write what I think is a very useful dispatch about the fundamentals of excavation and how we recover things buried in the ground. First of all, how do we keep track of all the artifacts we lift? It’s very simple really. One way to do this, which Alison has done at BOK-2, is to lay out a grid using string and nails at the outset of an excavation. That is, we divide the site up into smaller squares. The squares are each 2 meters long by 2 meters wide, and we use the Pythagorean theorem to ensure the grid has right angles. Who ever said geometry wasn’t useful? 

The grid is labeled in a simple manner, much like an excel spreadsheet. At BOK-2, the columns are labeled as numbers (e.g. 1-13) and the rows as letters (e.g. F-J). This system makes it very easy for us to communicate about certain findings. For example, “F7 has been a prolific square this season, but we’ve found hardly anything in H11.”

The above system organizes the excavation across the site (horizontally). But how do we distinguish the artifacts at different vertical locations, that is, in terms of depth within the sediment? For this we use a system of layers, which we call spits. As the crew digs their assigned square, they dig each square one spit at a time. A spit is only 5cm thick, and the spits are numbered as they go down. So, the closest spit to the surface is 1, whereas a meter deep would be spit 20. Therefore when an artifact is lifted it could be called “Square I6, spit 25.” This system of thin layers allows us to follow the changes in the sediment as we dig deeper at the site. Keeping track of the spits is useful for determining different points in time when artifacts were left or deposited at the site, since deeper artifacts are usually older. And we might also be able to recognizes different environments as you dig into deeper spits and older periods of artifacts.

two men holding a large wooden tray-like sieve and tilting the soil into another sieve below with a smaller diameter screen
Vincent and Benard sieve soil from the excavations at Site 15
As artifacts are discovered, they are left in place in the ground to be lifted with the transit (see Day 7 July 2). But as the crew digs, what happens to all that leftover dirt? It is not simply discarded! Every layer of soil removed must be sieved, or sifted, through a wire mesh. The sieve helps the crew find even the smallest chips of stone and tiny fossils. The soil is sifted by square and by spit, so that any finds can then be assigned to their position within the dig. If a certain square and spit happen to yield several small bones in the sieve, the sediment is packaged separately and sent back to camp to be wet sieved and searched for microfauna! We’ll talk about microfauna more on another day.