Day Eleven in Chaco

Marnina Patrick

10/19/2016

Today we arrived at the Wetherill Homestead with lovely weather conditions and unloaded all our gear. One student went with Jennie Sturm, a UNM Graduate Student, to gather data using ground penetrating radar.  Two of us went with our Teaching Assistant down into the Chaco Wash to do geomorphology work. We faced (used a tool to clean up and create a flat surface) a section of the northern wall where a hearth had been spotted. In a short time, we uncovered evidence of burnt organic material and after Professor Wills was informed of our find, we were directed to take samples for radiocarbon dating.

 

















Our main project for the day was to map the alluvial stratigraphy of the northern wall in the wash. Alluvial means relating to the flood plain and there is also fluvial which is relating to the channel waters. The southern wall has clear stratigraphy but the northern wall has a coating of clay from the run-off from above which is called a drape. We measured out a three-meter-tall and one-meter-wide spot on the wall and got to work facing it with our trowels. The higher area was worked with a facing tool and eventually a ladder was required to reach the top of it.  

Once our wall was faced and the stratigraphy was clear, we worked on meticulously describing each of the eight defined layers or strata. The categories of description are: texture, inclusions, bedforms, artifacts, changes in sediment, soil color, and sorting. For example, sediment is collected and water added to see how it behaves when wet to evaluate the sand to clay ratio in the sample. Inclusions may be either mineral or bioturbation so we look for evidence of iron or if roots are present in the layer.  As for the bedform, we look to see if there is lamination, which is how the sediment in that layer is laid down and we note if it is horizontal or on an angle. The soil color is determined by comparison to a standardized color chart in a Munsell book. To do this we place sediment under the corresponding color page and it has a window so you can compare the soil to the different color options. Lastly, we look to see how homogenous the sediment is and that is determined by how well sorted the different particles are.

What we were able to read in this stratigraphy was that there were definitely channel waters, and not just floodplan waters, at work here. The channel waters leave visible layers of sediment in typically not straight horizontal layers. Some were angled and some had curves or swales apparent. Also, due to the continuous presence of water there is evidence of iron with its tell-tale rust colored leeching into the surrounding sediment.

We also walked through the bottom of the wash, observing numerous examples of how water shapes our Earth.  Such as how a narrower channel forces the water to move faster with a higher velocity, which enables heavier sediments to be moved and deposited. Clay is easier to get moving but harder to drop and sand is harder to get going but easier to drop. We saw that around bends there were gravel deposits and the sand had definitive ripple shapes so heavier sediment was being moved by the faster moving water around the bend.

It was truly a beautiful day in the wash as there was not much wind and the Fall colors of the cottonwoods are striking. 

We returned to the work site as our fellow crew members were finishing up in their units. We loaded up the trucks and headed back to camp with all of us eager for showers and dinner. Lastly, a film crew from UNM was out documenting the unique projects we are working on, so stay tuned for possibly seeing more footage of us in action.