Clearing the Discovery Pit

June 30, 2012

The goals of the trip were to finish screening all bone-bearing matrix from the Discovery Pit piled on the overlying terrace and to clear unexcavated areas in the bottom of this pit so we can launch into virgin deposits on the south side. The cleared pit will provide space to toss overburden when we dig the next trench parallel to the Discovery Excavation. Hopefully, the skull will be there so that we can establish its condition and how far we can dig through the overburden without hitting any bone.  It's a real tribute to the volunteers who have worked this spring/summer that they've been willing and eager to endure the tedious work of this "cleaning up," especially at 100 degree plus temperatures.

 One rib and some chunks of a large limb bone, one with strange post-mortem marks, were recovered. The rib was standing almost on end, vertebral articulation up, and clearly was reworked from the primary deposit into the tributary cutting into the site. A rib recovered on the first excavation was similarly orientated and visible protruding from the stream, close to the limb bone chunks. During the mapping expedition, on June 24, a fragmented vertebra was collected just above the ‘standing rib’ and above the level of the bone bearing matrix. The ‘standing rib’ was visible when the vertebra was removed. How did the vertebra get above the level of the bone bed? The vertebra that John recovered with the backhoe on the first dig appeared to be above the level of the other recovered bones as well.  The definition of the bone-bearing unit is not clear at this point. 

The bone bed has been partially exposed at least twice previously. During our first excavation, historic ceramic fragments, remains of a modern calf and plastic debris were collected from mammoth bone-bearing fill in the Discovery Pit. Today (June 30) we pulled honey locust seed pods from two areas of the tributary, one under the nick point of the little creek and the other from dark matrix, also bone-bearing, previously regarded as the mother lode. These seed pods were better preserved than other plant macrofossils recovered and Dick Baker, UI-Geoscience, indentified European plant species in an associated matrix sample.  Thus, there are two historic lithologies associated with mammoth bone and modern plant macros. Primary deposits are present and exposed because the plant macro sample that Art collected in January was not contaminated. The bottom line - We do not currently understand the deposit.  

The two hotspots that Frank Weirich had previously identified with high resolution, 1.3 GHz GPR were probed and excavated but yielded nothing. The radar anomaly is apparently below the mammoth layer. Frank is working with his software vendor to get a glitch fixed and can't calibrate his maps right now with depth measurements.  He was tied up in another project this Saturday but will be back to try again the next time we go out.

There were 31 volunteers present,  representing the usual broad mix of MNH volunteers, OSA staffers, local residents/students and teachers, nature center naturalists and representatives from state agencies. Lee Wymore, Professor of Biology at Indian Hills Community College, Ottumwa, brought five students and some supporting staff members who spent almost the entire day with us.  Professor Jim North was back from Wm Penn, as ever, teaching down in the pit.