Eight volunteers descended on the Northboro sloth locality and worked all day on Saturday and dug through noon on Sunday. Sloth-bearing matrix next to the area where rib cage elements were recovered by Bob Athen was temporarily above creek level and removed about one meter of overburden over a four meter portion on the far (north) shore and started skimming for sloth. We also dug three test trenches perpendicular to the shore to get some idea of the depth of the bone scatter as well as geographic distribution of the critter. One femur was nearby. Video of the weekend by Tiffany will be on the Museum and Departmental web pages shortly.
This activity provided the following observations, prognosticates and revised work plan.
1. It is clear that the sloth is not completely articulated but that there probably are other packages of body parts as well as isolated bones in the deposit. Thus, we abandoned the grid-removal plan and started a more broad-based skimming strategy. We also tried to establish the depth of the bone-bearing interval.
2. The surface of the bone-bearing grey clay unit has been eroded contains a few small entrenched coarse sand stringers which predate the 1993 flood which exposed the sloth. There is at least 31 (34 below datum) inches of relief, much of which post-dates the1993 flood. All of the bones were recovered from 3 to 18 inches below datum, the thoracic vertebra being the deepest. The ribs were about 8 inches below the surface (11 below datum). We collected matrix samples to test for plant macrofossils, microvertebrates and pollen. Jim Huber has agreed to check our matrix samples for pollen.
3. The test trenches cut to 34 inches below datum revealed a major change to a coarser texture about 24 inches below datum. No bone was found in this unit. Thus, we believe (testing was limited) that the sloth is confined to the overlying blue-grey clay layer.
4. The stream bed, located south of the rib cage, is now 30-34 inches below datum and thus any sloth remains in that channel are now in lag deposits downstream. Two small bones were collected from the stream bed Saturday.
5. North of the rib cage concentration, the sloth layer is three inches below datum and some bones clearly lie on the surface of a disconformity overlain by later Pleistocene or Holocene deposits. There are at least two large bones exposed in the excavation wall in this direction. Fossils discovered here clearly have been exposed to weathering, are fragile and will require plaster jackets to collect. About10 feet of modern slump will have to be removed to reach these specimens.
6. Blue-grey pond deposits apparently thicken to the south and the best conditions for preservation are there. Any bone removed by steam erosion probably was trapped in a waist-deep pool about 20 meters downstream from the rib cage. A whirlpool forms over this feature when the water is high and this is a logical ware house for any bone removed by the present creek. The next priority is to drain and explore this pool.
7. We have arranged for an excavator divert the creek to the base of the first well-defined slump block about 50 feet to the south and bypass the pool and remove about 10 feet of overburden to expose 15-20 feet of the bone-bearing unit on the south bank of the present stream.
8. The excavator is scheduled for the end of next week (Sept. 17, 18 or 19) weather permitting. In order to divert the stream, the excavator will cut to the grade of the present channel and this will cut the bone-bearing unit if it extends to the ditch. The operator will spread this matrix over a broad area so that it can be searched for bone. Volunteers will monitor this process.
9. After this, we will pump the pool and search for sloth, probably by forming a bucket brigade of volunteers to pass the cobbles, gravel and presumably sloth bones to the surface. This should follow excavation asap because of low water levels. Dates will be announced shortly.
10. The next step, in the weeks ahead, will be exploration of the cleared area on the south bank. We may have an opportunity to explore by geophysical means before digging. We can tackle the north bank this coming spring.
11. University participants were Charles Monson, Lynette Stephenson, Tiffany Adrain, Dave Brenzel, Holmes Semken, JoAnn Seeman, Collen McHenry, John Hiebert and Katie Scanlon. Bob Athen, Sonia Athen, Dean Tiemann, Lorita Tiemann and Sonia’s father.
Holmes A. Semken September 10, modified Sept 24, 2003