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
A foray to the site August 14-15 netted 16 more segments of bone through the heroic efforts of 20 volunteers. Most of the elements appear to be portions of juvenile ribs. Positive assignment to the “toddler” or “baby” awaits cleaning and a closer look. A couple of unidentified pieces are intriguing, but time for cleaning and study was curtailed by the storms that moved through the region all day.
Holmes and I were accompanied on the trip out by Meghann Mahoney, UI Museum of Natural History and Jan Ailes, Education Facilitator, Indian Creek Nature Center, Cedar Rapids. We were joined in preparing the site for Saturday’s dig by our long-time Bobcat operator Will Mott, Council Bluffs, Bill Wiechman from the Greater Shenadoah Historical Society, and Mary Brenzel the co-PI’s sister, who drove up from Fayetteville, Arkansas to help.
14 intrepid volunteers joined the prep crew on Saturday, enduring intermittent rain and extremely mucky working conditions. Working until nightfall, they moved a mountain of clay and cleared an area of approximately 400 sq. ft. The bones were deposited on an undulating surface, which necessitates carefully shoveling through at least a foot of the dense clay matrix in their pursuit.
As Holmes wrote in his letter to volunteers last week, digging at the site is rapidly approaching its conclusion. It has taken 6 years, but we have now cleared a wide area surrounding the main deposit and only have a couple of narrow bands immediately under the berm left to excavate. We hope to take advantage of the seasonably low water levels and return to the site in a couple of weeks. Until we can discover the location of the Paramylodon that we know is nearby, it looks like we have reached the final act of the excavation phase of the project.
Thanks to the Des Moines area Central Iowa Mineral Society for organizing a large contingent of members for the dig. As ever, thanks also to the Iowa Archaeological Society, the Shenadoah-area community, Evans Rental, Glenwood, IA, which supplied the Bobcat and Will Mott, the Rembrandt of Bobcat operators. . . . Dave
Jan Ailes, Pam Belknap, Mary Brenzel, Harold and Kandyce Decuir, Charles Krauth, Meghann Mahoney, Lee McNair, Dennis Miller, Will Mott, Ed Peterson, Monica and Mica Post, Don Raker, Chuck Safris, Randy, Cheryl and Ryan Shanks, Marie Tiemann, Bill Wiechman, Holmes Semken, David Brenzel.
The week of July 6 was predicted to be sunny and apparently an ideal time for a two day excavation to recover additional remains of the baby and toddler discovered at the end of the May dig. As in the past, Evans Rental hauled the rented crawler to the site a day in advance. Will Mott arrived mid-morning with a large gas-powered pump which rapidly pumped the accumulated water from inside the levee. He estimated the capacity at about 100 gallons/minute. After the pit was ‘dry,’ Will relocated the levee protecting the previous dig north about 12 feet and then removed the muck resulting from two floods from off the floor of the excavation area. This was accomplished by 7:00 PM. Unfortunately, overnight thunderstorms produced heavy rains which lasted into the following day. While the creek did not overtop the levee, the rain was sufficient to flood the excavation and render the area inaccessible for the pump, crawler and people. More storms were predicted later in the day. The volunteers vowed to return as soon as possible after the area dried. Participants were William Mott, Lee McNair, Bill Wiechman, Aaron Last, David Brenzel and Holmes Semken.
Holmes Semken and David Brenzel, July 9, 2009.
It was susposed to be clear this afternoon but a big thunderstorm around 10:30 made the locality unworkable. We will start again as soon as it looks like we have a dry period. Sloth on, Holmes
Currently raining. 1/2″ last night shouldn’t pose a problem for the berm we built yesterday but it would be nice to have the ground dry out a little bit for walking around. Scattered lightning around us now too that has to move off before we go out. The forecast says all of this should be gone in a couple of hours and we can look forward to nice steamy afternoon of excavating. Will cleared a large area for us yesterday–hate to see all of his work wasted. Dave
Holmes and I are here in Shenandoah. We drove over last night from Iowa City with long-time MNH volunteer Aaron Last. Will be joined by dig-veterans Will Mott, Rob MacAfee and Lee McNair this AM. Despite an inch of rain on Saturday, the creek is still down and the forecast is good (chance of scattered showers tomorrow). Tuesday is scheduled to be an equipment day with most of the morning and early afternoon being spent by Will clearing the muck and moving the berm. Most of the volunteers are coming in Wednesday to excavate . . . . Dave
Since we started excavating in 2003, Holmes has been keeping an official log he calls Megalonyx Matters documenting our significant activites. This is publication #17, covering the preparation work leading up to our last expedition and its results. Lest anyone be concerned, the stubborn beaver discussed below is alive and well, and continuing to safeguard the site with a cover of at least a foot of water . . . . Dave
Megalonyx Matters 17: Site preparation (Aug. 27, Oct. 3, Dec. 4) and excavation (Dec. 6)
On August 27, 2008 Dave Brenzel, Holmes Semken, Phil Mather (Mather and Sons) and Bob Athen (landowner) met at the site and devised a plan to extend the sloth excavation about 30 feet into the south bank to search for more juvenile remains. The first obstacle proved to be a downstream beaver dam that substantially increased the water level over the bone-bearing, blue-grey clay. Bob had experimented with removing the dam by hand. This worked but the beavers repaired it overnight. It was clear that each excavation would be initiated by attacking the dam.
On October 3, Holmes and Dave returned to oversee the Mather’s levee repair and overburden removal. Dave waded in and breached the beaver dam by the time the excavator arrived (8:00 am); the water dropped 2.5 feet at the site. We were delighted because this exposed the bone-bearing matrix. Phil (name of operator), who operated the excavator, got the machine to streamside (and back out) via some creative road repair over very mucky terrain resulting from the infamous 2008 flood. By the end of the day there was a combination dam/entrance ramp both up and downstream from the future excavation area. The ramps were about 30 feet apart. The old lateral (midstream) levee was largely removed by flooding but it was traceable and was also reinforced by the excavator. Sloth bone, predicted to be under this levee, appeared safe. At the end of the day, the south bank was pulled back about 30 feet and the dig floor appeared accessible for a crawler to enter, clear remaining overburden (approximately one foot) and build a stream side levee to protect south bank excavations. Phil noted, “We now have a big hole to play in on the next dig.”
The crawler, operated by Will Mott, arrived December 4. Dave drove to Shenandoah a day early to reopen negotiations with the resident beaver. Will started clearing overburden by 9:30 AM and worked until nightfall. Dave and Will both noted the clay was more varied in color and, except for an occasional patch, less blue than previously seen. There was also a distinct up-tilt in the clay layer upstream (East). In the afternoon, after the bulk of the overburden had been moved, Will attached a blade he had fabricated to the bucket of the crawler and began shaving off thin layers of clay to reduce the amount the weekend recruits would have to remove. Dave stood by watching for traces of bone. At the end of the day Will used the bucket to dig a sump to collect melt water and satisfy Dave’s curiosity about the depth of the clay. He dug through approximately four feet of clay, all as homogenous as the surface layers, before striking a layer of fine white sand. The sand began to seep into the hole from the sides under the pressure of a slow flow of water that eventually filled the hole (so much for the sump idea). Dave and Will left the site by 6PM.
The Sloth Rapid Response Team journeyed to Shenandoah on Friday, Dec. 5th and they were on site at 8:00 Saturday morning. After bailing out the hole, and starting the pumps to keep up with Dave’s spring, the volunteers formed a line facing the stream side levee and began cutting a trench toward the south bank of the cut parallel to the levee. Spoil was thrown onto the levee. By the end of the day the area, except for a snapping turtle bone, proved barren.
While disappointing in terms of sloth bone recovery, the dig did define the south boundary of the bone scatter. This boundary is now defined to the north and east as well as to the south. Bone has been probed in the partly excavated intermediate areas where the juvenile bone has been concentrated. Another positive note: Ron Vogel performed a feasibility test on the clay using ultrasonic equipment he borrowed from his department. The results were promising and Ron hopes to construct a prototype of an ultrasonic bone detector in time for our next venture to the site. The next dig will occur after the spoil that forms the south levee is pushed back into the south excavation and a new levee created between the two entrance ramps. The crawler will again be required and we are working on ways to facilitate its access and egress.
Participants on the December 6, 2008 dig: Lynn Alex (OSA), Bob Athen (landowner), Cyril Below (grandson of Herb Dircks), David Brenzel (Co-PI, NSF grant), Andy Clack (Ancient DNA Centre, McMasters University), Herb Dircks (UI Rapid Prototyping Laboratory), Harold Decuir (President, Board of Directors, Greater Shenandoah Historical Museum, Kandyce Decuir (daughter), Pete Eyheralde (Naturalist, Mahaska County Conservation Board) , Elizabeth Fox (OSA), Cherie Haury-Artz (OSA), Sarah Horgen (UI Museum Natural History), Don Johnson UI Hospitals and Clinics, aka the Fossil Guy, and President, Eastern Iowa Paleontology Project, Meghann Mahoney (Museum of Natural History student-staff, Anthropology Major), Robert McAfee, Ph.D. (Faculty, Doane College), Holmes Semken (UI-Geoscience, Emeritus; PI, NSF grant), Austyn Slaybaugh CR High School student), Jennifer Sweet (OSA), Ron Vogel UI-Dept.of Physics and Astronomy), and Mary Weber (OSA).
Holmes A. Semken, Jr and David J. Brenzel, January 5, 2009
Ron Vogel, sloth volunteer and resident physics expert, has the theory that the remarkable uniformity of the clay that the sloth bones are resting in provides a unique opportunity to use ultrasound technology to search underground. If so it would save us a lot of time and money digging blindly for bones, and allow us to extend our search area significantly. Preliminary results are back from the clay samples he collected at the site last month and the results are promising.
According to Ron, “The results are. . . the same as I found in the tests on site. That is, the acoustic loss is about 5 to 10 times that of the human body. That seems like a lot but it is actually a lot less than anyone else has reported for soil. Based on these measurements, I’m working on a device we can use on site.” He doesn’t know if he can get it done in time to do us any good but plans to try.
Ground-penetrating radar, which uses radio wave frequencies, is generally used for remote underground sensing, but it won’t work at the site under our conditions–the wet clay absorbs too much of the signal. Ultrasound works similarly, but uses sound waves instead, and doesn’t suffer from the attentuation loss-problem under wet conditions. The technology runs into problems however with sorting out the meaning of the many different signal returns that result from trying it in mixed materials. For that reason ultrasound is limited to looking inside relatively uniform materials– like concrete roadways and bridges (for corrosion), or steel railroad tracks (for signs of fatigue or cracks) and of course people (for babies). More, as Ron moves ahead. . . . Dave
Participating in the dig: Lynn Alex, Bob Athen, Cyril Below, David Brenzel, Andy Clack, Harold Decuir, Kandyce Decuir, Herb Dircks, Pete Eyheralde, Elizabeth Fox, Cherie Haury-Artz, Sarah Horgen, Don Johnson, Meghann Mahoney, Robert McAfee, Holmes Semken, Austyn Slaybaugh, Jennifer Sweet, Ron Vogel, and Mary Weber. Thanks all for another fun and successful outing. . . . Dave
A lot of interest in the project in southwest Iowa. Here’s another story in the local paper:
The reporter, Tess Gruber Nelson, visited the site Saturday and called for an update Monday morning. KMA, the local radio station, called to do an interview too. Thanks again to everyone who participated and made it so memorable. Dave
Cleared the entire area (approx 10m X 2m) yesterday, a half-day sooned than expected. Unfortunately we weren’t slowed by finding any sloth bones. Recovered a single turtle bone. It’s a nice find though–our second species of turtle. Should aid in further defining the mysterious watery environment that left this 4 ft.-thick layer of remarkably homogenous clay. Ron’s ultra-sound tests gave him some promising data; he collected clay samples to take back to the university for further tests. With both the north and south borders of the creek cleared it looks now like we have just a couple of large areas in the middle of the creek left to excavate. We’ve probed for bones there and have confirmed their presence. We’re thinking about doing some prep work with an excavator in the next couple of weeks, before the ground freezes solid, and returning before the spring floods to finish digging. Photos next week. . . . Dave
After two years of delays due to high water and a couple false starts we’re back at the site this weekend with a full crew of volunteers–many veterans from previous digs. Here’s a link to a story published this week in the local paper.
Lynn Alex is back again with 4 other staff-members from the UI Office of the State Archaeologist (OSA). Ron Vogel is back too representing the UI Department of Physics and Astronomy. He brought equipment to test the feasibility of using ground-penetrating ultra-sound to locate bones. Herb Dircks is here with his grandson Cyril. Herb is a rapid protoyping consultant in the UI Engineering Design and Prototyping Center where he has been making duplicates of sloth bones to aid our research, using computed tomography (CT) scans supplied by the the Department of Radiology and Biomedical Engineering. Dr. Rob McAfee http://www.sloth-world.org/ is driving in again from Doane College, Crete, NE; and Don Johnson is also returning representing the Eastern Iowa Paleontology Project. Andy Clack flew in from McMaster University, Hamilton, Ontario, where he is studying ancient DNA under Hendrik Poinar in the Ancient DNA Centre.
There’s no way to thank everyone here for volunteering their time, in near freezing weather, to move the project forward. More tomorrow. . . . Dave
Holmes and I drove out to Shenandoah last week, and with Phil Mather’s help (Mather & Sons Construction) repaired the berm and opened up a new search area under the slump block on the south side of the creek.
There’s a stubborn but hard-working beaver just downstream from the site that isn’t taking the hint that it needs to move out. Bob tore the dam out twice last week before we arrived. The third time is a charm.
There’s rain in the forecast this week, but as soon as we can get in there again we plan to pump out the water and clear the muck from the floor with a crawler. Volunteers: With any luck we will put out a Sloth Dig Alert in the next three weeks. Keep your eyes peeled. Dave
We drove out to see the site Tuesday. The water level in the creek has dropped considerably in recent weeks owing to record-low rainfall in August. Bob thought we could start getting ready to dig again, but advised us to see the drastic changes in the topography since the floods earlier this year.
Phil Mather (left), Mather & Sons, who has come to our rescue a dozen times in the last 5 years with his excavating equipment and long experience; Holmes (center); Bob Athen (right), the landowner and discoverer of the sloth.
Looking downstream over the site. The floods straightened out all the curves in the creek.
Another view downstream and slightly north overlooking the dig site.
Looking upstream toward the north bank. The flood erased all traces of the road Mather built to access the site and the large hole we dug to reach the bones. Good thing we were finished over there any way.
We came up with a plan and hoped to resume digging in just a couple of weeks but a thunderstorm hit as we were leaving Wednesday morning and dropped 1.44 inches on Shenandoah–as much as 4 inches upstream. No telling how high the water rose. . . we may be stalled for a few more weeks. . . . Dave