Screening backfill & Radar Field Check

June 16, 2012

Thirty-six volunteers, primarily from William Penn University’s Division of Health and Life Science and the University of Iowa Office of State Archaeologist with help from University of Iowa Department of Geoscience, joined the museum establishment at the mammoth site. The primary objective was to screen previously excavated matrix, calibrate the Ground Penetrating Radar (GPR) maps made on a previous trip, and fine-tune the GPR by running a microgrid over a hot spot in the discovery pit. The volunteers signed waivers and examined selected bones that John brought for inspection. After a few minutes of orientation on mammoths in general, the group moved to the dig area, set up tripod screens as well as shaker screens borrowed from the UI Office of State Archaeologist (OSA) and began screening matrix previously excavated from the discovery pit. Foot bones had been discovered in the spoil piles and the piles needed to be re-examined. Also, the spoil piles were located on top of the area to be excavated next. Light brown backfill produced a few artifacts and some bison/cow bones, but clods from the grey matrix contained mammoth sesamoids. A neighbor of John’s saw the difficulty of screening the quantity of spoil so he went home and built a four by eight foot screen with a much coarser mesh. Since anything that we were looking for is at least the size of a walnut, this was more efficient for mammoth bone recovery and was much appreciated by the project leaders.

 Frank Weirich, of the UI Department of Geoscience and IIHR-Hydroscience and Engineering, scanned the area above one of the anomalies or "hot spots" discovered previously with GPR on a finer grid (one-half m) to be processed later.  John, using a loaned backhoe from Titan Machinery of Oskaloosa, [], then dug an exploratory pit over the anomaly primarily to calibrate the GPR maps and confirm the depth readings. At about mammoth level, several large rock cobbles were struck. As there is a boulder juxtaposed to mammoth remains in the discovery pit, the anomalies may indicate boulder/cobble beds located on the mammoth horizon. Subsequent analysis of the microgrid suggests that there may bone intermingled with the rock. This will be tested as we continue to systematically excavate the site. A thick sequence of blue clay-a pre-Gunder paleosol according to Joe Artz (e-mail communication)-was encountered above the cobbles. This coloration is typical of the deposits from which many ice-age mammals have been discovered in Iowa. There is still potential that the GPR will be refined to detect bone and Frank intends to refine the survey.

Another crew of volunteers excavated the remaining area of mammoth-bearing sediment around the discovery pit. One additional mammoth rib was recovered. Deposits abutting the dark black clay hosting the mammoth, while similar to the mammoth-bearing unit, are mottled in color and texture and contain historic stoneware (Ohio manufacture, 1905-Marlin Ingalls) and modern bovid remains. These probably are from a modern calf rather than bison. We initially thought that this was a slump deposit but Cherie Haury-Artz (OSA ) suggested that it was historic fill deposited in a previously cut valley, that was re-entrenched during the 2008 flood which also exposed the mammoth femur in the old valley wall. The density of the sediment suggests fill rather than recent slump.