GPS Mapping & More GPR
June 24, 2012
Frank Weirich and Frank Jr. used the GPR to map the terrace on the north side of the discovery pit, above the opposite bank previously worked. They plan to return on Saturday, June 30, hopefully with 3-D maps from the microgrid over the hot spot test explored on June 16 if they can get a vendor to fix a software bug. The Franks used their super-precise 1.3 gHz GPR antenna on a small area (approx 1 m. X 1 m.) down in the discovery pit directly on top of the bone-bearing clay layer. It doesn't penetrate as far as the 250 and 500 MHz so it cannot be used from ground level, but is accurate within mm down on the clay. They predicted location of two bones, although John then probed without results. Frank Sr. will take the data and process it to calculate the depth; John may not have probed deep enough, but it could be deeply buried rocks. We will test this area on Saturday, June 30.
Bill Whittaker, of the UI Office State Archaeologist, used his Total GPS equipment to capture GPS coordinates for the dig area, some previously excavated bones, Frank's GPR grids and a wide area of the surrounding terrain including the ravine and creek. His equipment uses a highly accurate GPS base station, a laser and a reflector that Sarah Horgen and Dave Brenzel carried up and down through the pasture, bushes, poison ivy and mud to measure distance, bearing, and height of any point that's in the laser's line of sight with an accuracy of a few centimeters. Bill will supply us with a very accurate map that we can use and add on all the GIS data that's available. Sarah contacted Don Wirth to get his map from the bones recovered on June 16th so we can integrate everything at hand into one official map. Don sent his map to Iowa City promptly and Museum staff will reconcile all geographic data.
John and his son probed the bottom of the discovery pit prior to the 24th and found another bone, broken in several pieces. Sarah took it back to the museum to be glued if possible. Digging with John Doershuk, they found three more ribs (in perfect condition) lying on top of each other which were then photographed and collected. Another bone, a badly weathered mammoth vertebral centrum, was recovered on top of a vertically-orientated rib. It could be the rest of the vertebra John gave Sarah to glue on the Inaugural Dig. At this point it is estimated that 20% of the mammoth skeleton has already been recovered -- not a bad start! The diggers were still amazed by the condition of associated plant material and a large sample of locust seed pods was collected to give to Dick Baker, who studies plant macrofossils in the UI Department of Geoscience.
Dave is impressed, as are all of us, that there are two ice age orphans/ghosts on top of the site-a honey locust and a hawthorn. Dave also discovered another classic orphan--an osage orange just 50 feet uphill to the west. We plan to show Dave Sedivec, Mahaska County Conservation Board, the next time he visits. It will be a terrific way to tie the mammoth to the kind of woodlands/ educational opportunities he could create at the Mahaska County's new nature center. Dave promised to bring the landowner some pawpaw and Kentucky coffee seeds on the next dig and plant them to add to his ice age collection.
Objective number one for Saturday, June 30, is to clear the rest of the mammoth-bearing clay down on the bottom of the Discovery Pit. There is room for 6-8 people to work down there. A few volunteers should bring their mud boots; the pit sprouted some springs after a half inch rain the previous night. It is an easy bail though. John left one rib in place for us to dig out. It is not laying flat but rather on its ventral surface. Sarah remarked that the way the bones are dispersed in "packages" and resting at odd levels and angles is strikingly similar to the sloth site, but not kicked around and mixed up quite as much. We need Art or Joe to look at the clay deposit.
If we have extra bodies the next time we dig, John wants us to screen the small amount of material sitting within about 1 m. of the ravine and kicked down into the bottom by the cattle. All the rest of the spoil piles can be ignored. John does not think that we need to screen excavated clay from the discovery pit, especially if we probe the clay and then shave. John is going try to get the backhoe back so we can throw matrix into the bucket like we did on the Inaugural Dig. Once we finish in the Discovery Pit we can start pulling back overburden lying immediately to the south and attempt to locate the skull. We will be able to shovel overburden down into the now empty Discovery Pit. More ‘sloth’ probes will help.