Tuesday, January 27, 2009
The Plant.
My company is doing a massive archaeology project at our county sewage treatment plant. The area is next to the Santa Cruz River (which has mostly dried up) and we are uncovering layers of prehistoric settlements going back over 3,000 years.
A prehistoric pithouse, the small holes are postholes.
Because the site is next to the river, it was periodically flooded, covering and sealing in settlements in sand and mud. After the flood water receded, people would return and build on the same spot, going back over and over again. Some of the settlements are now 10 feet below the modern ground surface.
Another house, this one with a large storage pit on one side.
The houses from this period, roughly 1200 to 800 B.C. (2800 to 3200 years ago) were made by digging round or oval pits in the ground, cutting small postholes, putting in saplings in the holes, and then basically building an upsidedown basket with saplings and reeds, coating the outside with mud on top to make them more waterproof. They are finding small burned patches in some of the houses, which are where a heating or cooking fire hearth was once present. Lots of storage and cooking pits around the houses. Many pits are bell-shaped, wider at the inside bottom with a narrow opening at the top. These sort of pits were useful for storing food and seeds, since the narrow opening made it harder for water or rodents to get into the pits.
3,000 year old field, click picture to see in more detail.
Perhaps the most exciting find has been a prehistoric field system. In the picture above you can see a small canal running diagonally along the right side of the photo. To the left is are the remnants of small berms, outlined in white paint, that surrounded the field. The canal water was diverted into the field through the opening in the berm, and the raised earth around the perimeter helped hold the water in place while it sunk into the ground. At the opening in the berm, next to the canal, two red flags mark postholes which once helped hold a gate or sluice in place. This could be lifted to flood the field, and replaced so that the canal water could be sent on to other fields. The white flags mark small planting pits, usually filled with a darker clay, where individual corn plants once grew. This is probably the oldest field system ever found in the United States.
Canal profile.
We cut trenches across the canals so that we can study the soil laters inside. Here you can see the distinctive broad U-shaped sediments that accumulate in the bottom of canals. This canal is somewhere between 2,500 to 3,000 years old.
I'll be spending the day out at the site on Thursday. When they turn the sewage mixing machines on, goodness!
Safety first!
