STPs are pretty small, 30 cm diameter, and are dug in 10 cm levels, with all the spoils run through the screen to catch any material you might not have seen while digging. We dug them in transects across the sites at 10 meter intervals. They are a way to get the bare minimum of information about subsurface desposits at a site - a veteran archaeologist friend of mine always refers to STPs as "peep-holes into the past."
We had a geologist do a geomorphology study of the project area, and knew from that work that the Holocene deposit in the area where we would be likely to find archaeological deposits was actually quite thin - maybe 30 - 40 cm at most.
The Holocene layer is underlain by a buried late Pleistocene soil that is 30, 000 years old or older. It has a distinctive orange color you can see in the picture above. You can also see how shallow it is. We ended up putting a little over 300 STPs total into the seven sites.
To be perfectly honest, our STP exercise was pretty much a bust, with only 8 STPs out of 310 showing any results at all. These were all isolated single flakes at shallow depths and none showing any evidence of cultural deposits. That didn't surprise us too much, though some people at the agencies issuing permits for the project had thought otherwise before we started work. The results will limit the amount of mitigation work our client will need to do to procede with the project.
We did see, or actually revisit, a lot of interesting stuff on the surface of the sites. For example, the big rim sherd in the picture above comes from a constricted mouthed jar. Its characteristics keyed out to the Colorado Beige type, that dates from the Patayan I period, AD 700 - 1000. It was likely made in the Colorado River Valley about 25 miles east of the site.
This is an Olivella shell bead. California archaeologists have come up with a comprehensive bead classification system to describe the bead styles. In this system this spire lopped bead is known as an A1e. Many of the bead styles have been dated from radiocarbon assays in sites (like ceramic or projectile point styles) and serve as useful "index fossils" for dating sites. Unfortunately people starting making the A1e style about 8,000 years ago and kept on until European contact so it's not very useful for that purpose.
This dart point base comes from the Pinto series, and dates between 3,000 and 5,000 years ago.
Most of the sites had grinding implements for seed processing, like these two metates still sitting on the desert pavement on one site.
Though it was late October, it was still over 100 degrees every day. We rigged up this shade so the crew could get a break from the sun at lunch.