Expedition Report, September 11-13.
Dave had Holmes in a car at 4:00 am and remarked that the latter was somewhat coherent in contrast to previous early departures. They met Will Mott at the site at 9:00 and set out with the crawler/backhoe to relocate the stream side of the existing levee to a partially submerged outcrop of previously excavated blue-grey clay. This left an oval-shaped lens of virgin territory which was scraped of overburden for excavation the next day. Thunderstorms were predicted overnight and the new levee pinched the Tarkio sufficiently to raise the stream depth three inches locally. To avoid stressing the fresh levee, the ramp into the site was breached to redirect part of the Tarkio into the south excavation. This was the reverse of when the creek was directed into the north excavation for sloth recovery on the south bank. Stream level dropped but the maneuver isolated the excavation area in the center of the creek.
The Sloth Rapid Response Team was ready at 7:30 am on Saturday. Bob provided a plank for a bridge to access ‘Brenzel Isle’ and the four foot drop to the stream below looked intimidating to many volunteers. All crossed safely and were comfortable with the span by noon. We did notice fewer trips to the picnic area on this dig. Although it rained overnight, the rain gods smiled on us and kept thunderstorms downstream until nightfall. The volunteers lined up four abreast at the up, down and streamside areas of the pit and began working toward the center. The area was cleared by 3:30 in the afternoon. No bones were found, but we encountered sediments unlike anything uncovered previously. Thus, the excavation produced a wealth of new information about the depositional history of the site thanks to the efforts of geoarchaeologist Joe Artz, Geospatial Program Director in the Office of the State Archaeologist. Joe specializes in sediments from archaeological (and now paleontological) sites. Since bones were not forthcoming, the crew of enthusiastic volunteers dug several deep trenches into the sediments for Joe to examine. He used the walls like a professor with a blackboard tracing the evidence for post-sloth erosion and deposition in the Tarkio Valley. Slight color variations of the sediment and subtle differences in sand and clay content confirmed that the blue-grey clay, largely a product of placid water, had been entrenched on at least three occasions in this area of the dig, Joe’s observations are appended below. The youngest of these units produced a shard of European glazed pottery and documented historic deposition. We apparently, as expected from the dig location, dug into the original creek bed and the previously flooded north excavation floor. One of the other channel deposits is partially conformable with the blue-grey clay. The other unit could be either immediately post-mortem of the sloth trio or later. Joe’s observations are appended below. While it was disappointing to not find any bone, the dig demonstrated that the excavation trenches closed on each other and that no bone was left behind. The postdepositional channels entrenched into the “slotheriferous’ blue-grey clay explain that missing elements of the three sloths may be related to physical as well as biological taphonomic processes.
Participants were Joe Artz, Holly Berg, David Brenzel, Jim Brownlees, Sallie Brownlees, Diana Dewaele, Robert Dewaele, Pete Eyheralde, Katie Eyheralde, Emily Eyheralde, Jaci Falco, Cherie
Haury-Artz, Sarah Horgen, Feather Lacy, Meghann Mahoney, Holmes Semken, Thalia Sutton, Ron Vogel, Don Wirth.
Holmes A. Semken and David J. Brenzel, September 18, 2009
Description: Stratigraphic Units Encountered on the September 12, by Joe Artz, Office Iowa State archaeologist. We encountered three stratigraphic units yesterday. For convenience, I’ll call these SUs 1 through 3, in order of ascending age. All three are channel facies, meaning that they have sedimentary characteristics of having been deposited by swifter currents of water than were encountered in the blue clay where the sloth remains were found. For convenience, I’ll call these SUs 1 through 3, in order of ascending age.
SU3 is a channel facies that is perhaps correlated with the sloth-bearing (slotheriferous?) blue clay. The blue clay represents a slackwater facies, where clayey sediments settled from suspension in a low energy environment with only very slight currents represented by micromorphologically visible laminations and oriented sand grains. In SU3, the blue-gray colored sediment matrix has sufficient very fine sand that it will not ribbon. There are also macroscopically-visible, discontinuous, laminations and lenses of fine to medium sand with rare pebbles. These materials were deposited near, and possibly within the stream channel.
SU2 overlies SU3. It is a grayish brown silt loam that is finely laminated. In the south half of the island, SU2 has an abrupt, uncormable boundary with SU3, and appears to fill a small (ca. 1.5 wide by 30 cm deep) trough incised into SU3. This is perhaps an overflow channel scoured by floods and filled with more oxidized sediment that was probably reworked from a nearby, better drained sedimentary facies. The abrupt SU2-SU3 contact fades to the north in the profile, and in the north part of the island, has a conformable contact with SU3. This suggests that SU2 may be contemporaneous with the upper part of SU3. The SU2-SU3 contact trended northwest across the excavated surface, and seems to have been thicker in the west part of the excavated area of the island.
SU1 was encountered along the north side of the island. It is a laminated blue gray loam or very fine sandy loam. It is similar to SU3, but sandier, less consolidated, with more distinct laminations in the lower part. It yielded finely-divided flecks of bone and small bits of wood, and Euroamerican ceramics (undecorated whiteware). The latter suggests a historic channel fill deposit, perhaps of the 1917 excavated channel, or a post 1917 channel. At the south end of where we’d excavated, SU3 did have a very abrupt contact with what looked like SU2, which clearly indicates a historic age.