Monday, 22 April 2013

Geoarchaeological Adventures Across the US Part 2: Chaco Canyon, New Mexico


Following on from my post a couple of weeks ago, here is another snippet of geoarchaeological observations from my recent road trip across the US. As I mentioned in the previous blog, the coast to coast trip was a great chance to observe the gradual changes in geology, vegetation and climate that happen across the continent, and how these link with the archaeological preservation. In the humid subtropical climate of the Florida pan-handle we saw a lot of water action and salt precipitation occurring at Fort Pickens, with relatively degraded brick structures that are only around 180 years old. The contrast in New Mexico is significant. The climate here is semi arid to arid, and the preservation of the brick structures (about 1000 years older than those at Fort Pickens) in the Chaco Culture National Park is fantastic. However we do still see other types of weathering occurring. Whereas most people would be taking pictures of the fantastic structures (actually, I did that too...), I was taking pictures of different types of honeycomb weathering. This is a type of salt weathering that occurs on sandstones and other rock types under certain conditions, requiring a combination of wind and salt exposure. Salt is deposited on the rock surface by wind, and when the moisture evaporates the salt crystallises within the pore spaces, causing the grains within the sandstone to disaggregate. Repetition of the wetting and drying process causes the pores to gradually become larger, creating rock cavities of varying size which are called tafoni.

Although the majority of the buildings at Chaco are made of sandstone bricks, there are only certain areas where the process seems to occur, and it also occurs on different scales. I am guessing this has something to do with the level of wind exposure. The photographs below show a few examples of this type of weathering near Pueblo Bonito, the largest of the housing complexes in Chaco Canyon.

For more on honeycomb weathering including a great bibliography check out this website: http://www.tafoni.com/Bibliography.html




Thursday, 18 April 2013

Micrograph of the Month: Pseudomorphic Voids

Plant voids in different materials in thin section
I mentioned these features briefly in a previous post on wall plasters - these micrographs all show pseudomorphic voids in different contexts. It would actually be technically correct to call these pseudomorphic plant voids, as there can be other 'pseudomorphs' found in geological thin sections, which occur where a void/space originally formed by one type of material is replaced by another type. This leads to the crystal shape being that of the original mineral rather than the mineral that is currently present. Pseudomorphic plant voids in an archaeological context occur in materials where plant remains were once present but have now decayed. This leaves an 'impression' of the plant remains, which can give important clues on the use of plant materials in archaeological contexts.  It gives us a different insight to that of the charred macrobotanical record. Materials such as those shown above may not contain plant remains in charred form, and looking at these types of voids may be the only way of determining the type of volume of plant remains that were used for tempering clay materials, or that were being consumed. In some cases we can combine this with phytolith analysis, though in the materials I have examined the presence of phytoliths within the voids is suprisingly rare. In fact I have only seen one example, shown above (upper right), where you can see a grass leaf/stem phytolith within a void in mudbrick. In the upper left you can see seed shaped voids in calcitic plaster (maybe an archaeobotanist could hazard a guess at the type of plant?). In  the lower left is a great example of how the angle of the cut impacts what we seen in thin section, in this case showing both transverse and longitudinal voids in grass tempered clay aggregate. In the lower right we can see an omnivore coprolite with plant voids.

Tuesday, 9 April 2013

Geoarchaeological adventures across the US part 1: Pensacola, Florida

Degraded brickwork at Fort Pickens, Pensacola
Limited blogging from me over the past couple of weeks, due to a lack of internet rather than a lack of activity. In fact I've been incredibly busy with archaeologically and geologically themed adventures, travelling from Pensacola (Florida) to Powell Butte (Oregon) via Texas, New Mexico, Utah and Idaho. A 5 day drive across a major chunk of the US, it's been amazing to see the changes in the landscape, geology and climate, some of them gradual and some more dramatic. One thing that is particularly interesting from a geoarchaeological perspective is the difference in preservation of different sites that we've visited, related to the local environment.

Salt stalactites


Here for example we have Fort Pickens, a historic monument that was contructed in 1829, completed in 1834, and remained in use until 1947. Despite being quite recent in archaeological terms, much of the brickwork has started to degrade and/or has been buried by sand. The fort is part of the Gulf Islands National Seashore park and is located on Santa Rosa Island, a huge barrier island 30 miles east of the Alabama border. The level of salt crystalisation from sea water washing through the brickwork is impressive, and I imagine it's a bit of a nightmare from a conservation perspective.

This contrasts significantly with another brick heavy site we visited - Pueblo Bonito in New Mexico. Pueblo Bonito is part of the Chaco Culture World Heritage Site. Chaco dates from 850AD and the preservation is just amazing, though there is still a degree of weathering going on at the site, related to salt crystalisation of a different sort. More on Chaco coming soon.