This weekend I finally got around to putting together my various notes from database and GIS crunching and field observation on the Western Argolid Regional Project. Since we’re still working to analyze finds from this season, our main body of data derives from artifact densities. That being said, we have been able to spend a little time figuring out what variables had the greatest influence on artifact recovery throughout the survey area.
Visibility. The overall visibility in the survey area was right around 50%. Surface visibility did not correspond with artifact densities in a linear way, as survey archaeologists have come to expect. The highest artifact densities peaked first in units with 50% visibility and then in units with 70%—90% densities before dropping off in units with 100% artifact densities. In fact, units with 100% visibility produced fewer artifacts per ha then the average for all units. This serves as a useful reminder that visibility and artifact densities are independent variables even if the drop in density at 100% visibility hints that something strange must occur to artifact recovery rates in fields which have been finely plowed and cleared of all vegetation.
Surface Clast Size. We also recorded surface clast size for each field. Most of our fields consisted of 19-75 mm coarse gravel and these fields along with those with cobble sized (>75 mm) surface clast produced the highest densities. The average visibility in these fields falls between 41% and 53% respectively. Cobbles tended to produce more artifacts per ha than average visibility alone might suggest, but not by a vast margin (1040 artifacts per ha rather than the 913 artifact per ha that units with 50% visibility tend to produce). Units with coarse gravel were consistent with visibilities. Interestingly, units with fine gravel or sandy soil produced fewer artifacts than their average visibilities would suggest. Sandy soils, although relatively rare, had 41% visibility but produced only 390 artifacts per ha. It’s tempting to see sandy soils as recently deposited riverine sediments, but they don’t necessarily pattern that way across the survey area.
Background Disturbance. Recently, survey archaeologists have begun to think about background disturbance as a major influence on artifact recovery. This term describes the amount of objects in the soil matrix that distract the eye from the ceramic and man-made lithic objects we are supposed to be identifying. We recorded background disturbance as either light, moderate, or heavy (or none). Our data showed that units with moderate and light background disturbance performed more or less consistently with their visibility. Units with heavy, background disturbance, however, had much higher than average visibilities (70%) and much lower than predicted artifact densities than this visibility alone would predict. This suggests that high background disturbance might influence recovery rates in a substantial way.
Dominant Vegetation Height. For each unit we recorded the dominant vegetation height. This correlated strongly with surface visibility – as one might expect – with densely overgrown units with vegetation head high or higher (!) having average visibility in the teens (18% and 17% respectively), and waist high vegetation averaged a paltry 33% visibility. Interestingly, head high or higher vegetation produced lower artifact densities than suggested by visibility alone, but we’ve long reckoned that our visibility scale runs to imprecise with very low visibility fields. Units with vegetation at knee height coincided produced densities that coincided with expected visibility, but units with ankle height vegetation produced more artifacts than one might expect from visibility alone.
These short studies demonstrate that artifact recovery rates are influenced by a range of variables present in the landscape. Using visibility and artifact density as a baseline for understanding artifact recovery allowed us to recognize the influence of a range of variables that impacted field walker performance. The highest recovery rates appear to come from units with cobble or coarse gravel, ankle high vegetation, plowed, loose soils, and light or moderate background disturbance producing visibilities of between 70% and 90%.