Geological Assault, Excavation and Second Right Scapula

July 20-22, 2012

On Friday, July 20, a small crew from the Museum of Natural History and the Department of Geoscience descended on the site to conduct geological exploration of the bone-bearing deposits, clear the Discovery Pit and test new probes, funded by Frank Weirich, especially made for the excavation Essex Rock Bit of Monroe, NY. Additional geological analysis was essential because the depositional situation is not clear. The Discovery Pit was attacked but efforts to complete excavation of the area were not finished.  Since then, Farmer John has uncovered mammoth bones 18 inches below the level previously deemed to be the bottom of the bone bed. The thickness of the deposit, at least under the modern deposits in Bone Gulley (previously called tributary), is much greater than anticipated .Perhaps Farmer John was excavating in an older channel of Bone gulley channel. All of the bones discovered so far have been moved from their original position, initially as slump blocks which would not sort skeletal elements. Subsequent fluvial activity, some of which is historic has either cut into the bone-bearing slump or undercut the unit which then collapsed into Bone Gulley. As yet, there is no indication of the direction of the source. Also, as noted previously, the specimens were coming from or are surrounded by two historic deposits, possibly facies of each other, but likely distinct units. As previously reported, one contained historic ceramics, plastic and calf bones. The other did not have historic artifacts but Dick Baker, Geoscience, found invasive plant macrofossils. How often has the bone bed been modified over time?

The Friday crew left Iowa City in time to arrive at the site when Farmer John got off work at three o’clock. We were early and John found us in the café.  Dave drove with him to Titan Machinery to borrow the backhoe for the weekend. John then cleared a bench seven feet wide (backhoe’s reach) down to about two feet above the bone bed and smoothed the surface for Saturday’s GPR surrey closer to the bone bed. Sarah Horgen, Andrew Blodgett, Museum Natural History student/staff, and Ryan Bozer, museum volunteer & West High student, began draining the Discovery Pit and shovel-testing small areas of the Discovery Pit floor. Tom Jorgensen, University writer/photographer reviewed our operation and took pictures for theSpectator, an e-magazine for UI staff and alumni. Art Bettis and Phil Kerr, UI Department of Geoscience, shaved the nick point section, the only undisturbed sediments exposed, and hand cored the subsurface sediments to a depth of almost five meters. They bottomed in sand that was under sufficient hydrologic pressure to induce flow into the Discovery Pit. This is the local groundwater that is capped and confined by the silty sediments that are part of the upper layer yielding the mammoth remains. On Sunday, Art and Phil returned to take two cores, one on the high ground 10 meters southwest of the Discovery Pit and another about 50 meters almost due west of the Discovery Pit. Art and Phil’s coring documented a consistent stratigraphic succession of fine-grained pre-Gunder alluvium grading downward into sandy alluvium that is more than 5 meters thick.  The zone where the fine grained deposits grade into sandy alluvium contains abundant organic matter and plant fossils and is the same layer where organic remains crop out in the lower part of the knickpoint profile in the Discovery Pit. Art also verified that we are not digging in primary deposits but in reduced, reworked, sediments. The bones were originally upslope, in as yet to be defined direction.  We completed Friday’s work around 6:30 PM. Excavations on Saturday and Sunday ended between 2:30 and 3:00 PM because of the extreme heat. The discovery Pit still is not cleared.

The previously cut bench in the overburden to reduce overburden thickness above the bone bed to two feet also provided a flat platform to refine the GPR analysis. Frank dragged this bench with two antennae (250 &  500 Mhz) in an area where bone is anticipated and will be exposed over the weekend,  Frank returned Friday with his highest resolution antenna GPR antenna (2.3 GHz) to explore the floor of the Discovery Pit. On Sunday, Frank, along with Geoscience graduate student, Megan Schettler, used a 5 cm grid drawn on a two meter square plastic sheet for a level base to scan the exposed floor of the Discovery Pit. The subsequent discovery of a second partial scapula and two ribs under the bench and several small bones under the Discovery Pit floor should permit detailed evaluation of the effectiveness of GPR at the mammoth site.  Hopefully, both Art and Frank will be able to integrate their findings before the next excavation.

A large body of volunteers arrived both Saturday and Sunday. Those on Saturday were largely from the Cedar Rapids Gem and Mineral Society and those on Sunday were primarily provided by the Indian Creek Nature Center in Marion, Iowa. The latter arrived as a morning and afternoon shifts. Working in the Discovery Pit and underneath the bench, volunteers uncovered a second partial right scapula (We have two Individuals!), two ribs, several foot/ankle bones and segments of a deformed tusk over the course of two days.