Thanks to Thalia for sending in these photos of the September excavation. Lots more in Flickr.
A 12,000 year-old mystery in SW Iowa
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Joe Artz sent us the following report concerning the clay layers we observed Saturday. A 10,000 B.C . or early 20th century flood shortly after the stream was straightened (ca. 1917-1923) would explain why we didn’t find any bones in what appeared to be such promising ground.
We encountered three stratigraphic units (SU’s)–for convenience I’ll call these SU’s 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.
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, unconformable 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.
I’ve long pondered the improbability of 4 ft. of clay accumulating without a break–500 years (?) without a flood?! It’s comforting to find some evidence finally that they were indeed occurring. I expected a more dramatic signature–sand and rocks–but Joe says the small differences seen here aren’t unusual for western Iowa at this time. Flood water transports what’s available, and several thousand years of wind-borne deposits left a thick unstable blanket of loess in the region. The hills literally melted away with the rain and spring thaws and for a long time the Tarkio was thick with loess sludge and not much else. Thanks Joe for a vivid new picture of the Valley. . . . Dave
The rain gods smiled on us all day Saturday, keeping thunderstorms west of the site and downstream until nightfall. No bones were found, but good fortune located the dig in a section of the ancient Tarkio streambed unlike anything we’ve uncovered previously and the excavation produced a wealth of new information about the history of the watershed thanks to the efforts of geoarchaeologist Joe Artz, Geospatial Program Director in the Office of the State Archaeologist who assisted us on the dig.
A large crew of enthusiastic volunteers cleared the target area quickly and then dug several deep trenches into our clay deposit for Joe to examine. He used the walls like a professor with a blackboard tracing the evidence of the long and often violent history of erosion and deposition in the Tarkio Valley.
Slight color variations of the sediment and subtle differences in sand and clay content confirm a story that contrasts sharply with our long-held impression of placid water in the sloths’ valley. The evidence points to multiple floods cutting deeply into the clay on many occasions.
The Tarkio Valley has often been called a “hungry canyon” for its soil-devouring proclivities, but who would have guessed that history went back over 10,000 years! More photos and Joe’s preliminary report tomorrow. . . Dave
Everything is going according to plan. The water level is about as low as we’ve ever seen it. Will cleared a lens shaped island in the middle of the creek Friday–approximately 30 ft. long and 15 ft. wide. Work went fast, we’re out from behind the sand bags and didn’t have the usual 15 inches of muck to tend with. We’ve dug up part of this area before but sections were covered by the berm and look promising. Bob Athen is optimistic. 30% chance of rain later today. Photos next week. . . . Dave
We have the crawler reserved for this Friday to prepare the Tarkio Valley site for a dig on Saturday. This could last into Sunday (but not likely). We will meet at the Days Inn parking lot at 7:30 on Saturday September 12 and caravan to the site. We will provide water, sunscreen and Deet. You should bring your own light tools (trowel, etc.). Dave, Will and I believe that we have dug up to the north bank excavation cleared a few years ago but that bone may still lie below one of our old levees. This area will be the target this trip. Will should have the overburden cleaned down to this level by Friday afternoon. Sloth on. Holmes