Tuesday, June 29, 2010

The Joys of Ground-Truthing

In past posts I have noted the importance that geophysical survey has played in our project. From the initial magnetic survey carried out in 2008 to our efforts to "ground-truth" the results by excavating where significant magnetic anomalies have been recorded, magnetic gradient survey has proven its value many times over. Its most spectacular success was the detection of the pit house, Feature 09-04, last season, but this year's work reveals how truly sensitive the instrument is, and, of course, the great proficiency of its operator and data-cruncher, Jarrod Burks.

In this post, I provide some of the results of our ground-truthing for a quite busy section of our current excavation area. The image below is a closeup view of the magnetic survey map for the western section of our bulldozer transect. The dark anomalies with feature numbers are those that mark prehistoric features. Note that every significant anomaly turned out to be a feature! No duds in the bunch. Pretty darn good and a great help to our efforts.

Probably the most indistinct feature is 09-18, a section of the oval enclosure ditch. It contained little of the oxidized soils, pottery, or fire-cracked rock that make for a strong anomaly. And it wasn't very easy to see when we dug into it! Feature 09-14 was a large but very thin lens of midden soil, charcoal and patches of burned earth. Still, the two small decorated pot sherds it contained proved very useful for placing its origin in the Late Prehistoric period. In contrast, Feature 09-26 was a large and deeper pit that contained nothing but charcoal, FCR, and a few flakes--very uninformative by comparison. Feature 09-34, shown below, was small but magnetically powerful since it was filled with fire-cracked rock.

Feature 10-13 was just opened today. It appeared as a rather indistinct, gray-brown feature stain in plan. Excavation of one half revealed a healthy amount of FCR, pottery, and charcoal-laden soil. A respectable amount of magnetic stuff but seemingly not that different from other features we have found that did not show up on the mag map. But we still have the other half to dig, so we will see.

Finally, there is what I have been calling "the big blob," a very large, somewhat irregular mag. anomaly, the north half of which lies within our transect. Now I suspected that this was more than one feature, but upon excavation it proved to be very complex. What we found were at least three overlapping pit features with some dark soil zones between them. We have spent the last several work days carving these features up in such a way as to sort them out. We are still

at it, but at least one we now know dates to the Middle Woodland period. I will have more to report on these interesting features later, but for now they reveal how complex the relationship is between the archaeological record and our high-tech methods of reading it.