Friday, January 15, 2010

Grains of Sand

For archaeologists, soils brim with information. During our initial training, we learn how to glean some of that data and make sense of it. But lots of archaeologists lots of the time throw away useful information with each shovel full of soil that falls through the screen.

Grains of sand, for example, can be classified and the sediment of which they are a part can be interrogated. Where did the sediment come from? How was it modified? How did that sediment play a role in the human use--or avoidance--of a particular landform.

A very useful and remarkably inexpensive tool rarely seen on archaeological sites is the sand grain size and shape chart. This handy little tool folds up to the size of a wallet--and, unfortunately, as thin as many wallets-- and conveniently fits into the jacket pocket of a Munsell soil color book.

The chart illustrates sand grains of different sizes, allowing the field worker to describe the material as fine, medium, or coarse. Examine sediments under magnification of, say, X10 (ten diameters, or ten times the actual size), and compare the grain shapes to the Roundness chart. Large, angular grains probably haven't been transported very far, while small, spheroid grains may have been carried great distances. Large (coarse) grains probably were deposited in a high energy, high velocity environment (wind or water) while finer sediments settle out in lower energy environments.

As a case in point, there are many relict sand dunes in northern Anne Arundel County and on Maryland's Eastern Shore that formed several thousand years ago during a stretch of dry weather. The sand grains that dominate these deposits tend to be coarse and well rounded, a product of wind erosion in an increasingly arid environment. When those winds encountered any obstruction--an existing dune, a clump of trees--the obstruction sufficiently sapped energy from the wind to allow the heavier particles to precipitate, forming an elongate sandy rise that continued to accumulate additional material. These typically appear on aerial photographs and topographic maps as crescent-shaped rises aligned northeast to southwest. They were favored occupation sites for many Archaic period peoples, probably because they typically were associated with wetlands with rich resources.

Grain size and shape in the various deposits encountered at Port Tobacco also can reveal information about how they formed and, by extension, how the various landforms in and around Port Tobacco were created and modified over the millennia.


Wednesday, January 13, 2010

Color me 10YR5/8

At any archaeological site it is absolutely crucial to analyze the soils as well as the artifacts. Investigating the soils can reveal how different strata formed and how processes such as erosion shaped the landscape. This is especially important at Port Tobacco, where the movement of soils and the silting up of the river had a major impact on the town. In the field we record the depths of different soils and note their color values. Now, without some sort of color standard we would end up taking a look at excavation notes and finding all sorts of color descriptions...with names like "brownish red" or "mouse brown." While these sorts of descriptions may be somewhat accurate, they are quite subjective. How do I know that what I consider to be light brown is the same thing you consider to be light brown? What if I have two different shades of light brown? Since everyone sees colors differently, it would be near impossible to create a clear set of soil descriptions for an archaeological site...but no need to fret! This is where the handy-dandy Munsell book becomes our reference (a tool of sorts) of choice.

This brilliant blue book contains 322 color chips on 9 different charts, identified based on hue (a color's relation to red, yellow, green, blue, and purple), value (lightness or darkness of a color), and chroma (the purity of a color). Created in the early 20th century by an artist and professor named Albert Munsell, it was adopted by the United States Department of Agriculture in the 1930s as the golden standard for conducting soil research.

When reading a page in a Munsell book, the vertical numbers denote the value of a color, while the horizontal numbers specify the chroma. The hue is designated by what page you are on. For example, the Munsell notation of a particular soil is 10YR5/8. The color name of this notation is yellowish brown. The YR is the abbreviation for yellow-red, and the 10 refers to where on this particular letter range (yellow-red) the color is. The 5 represents the value, and the 8 is the chroma.

In the field a soil should be matched to the most similar color chip, and its texture and any inclusions should be noted. The best way to compare colors is to either hold a bit of the soil behind each cut-out hole by a color chip, or simply by holding it next to the chip.

While there will still be a little bit of variation in how people see the colors of soils, the Munsell book established a universal system for describing colors that gets us much closer to an accurate soil description of a stratum. The book is also handy for estimating proportion of mottled soils and looking at a soil's granular structure. Munsell books are not only used for soils, but are a standard for describing hair and skin colors in forensic pathology, as well as describing colors of beer in breweries! Someone has to make sure that amber ale is actually amber!

So, when on a site learn to love the Munsell book. Identifying soil colors quickly and accurately requires practice, practice, and more practice!


Tuesday, January 12, 2010

Heidelberg Invades Florida

The indomitable April Beisaw led a large group of Heidelberg University students to the Society for Historical Archaeology conference at Amelia Island, Florida, last week. They drove for nearly 18 hours through snow and ice to get there.

Not nearly enough instructors bring their students to professional conferences. The setting provides students the opportunity to meet prominent people and young up-and-comers like themselves in a collegial environment. Kudos to Dr. April and Heidelberg University.

Port Tobacco participants may recognize a couple of the ladies: that's Magen in the red coat at the far left; Theresa in the purple sweater third from right, front; and Amanda standing next to the inestimable April. The tall fellow in the back is Dr. David Bush who for many years has involved students in all facets of archaeological research at the Johnson's Island prisoner of war camp, a Union facility on a small island in Lake Erie.

I expect we will see many of these same faces at future academic conferences.


Sunday, January 10, 2010

Get the Point?

Last week's Mystery Artifact was identified by newcomer ComradeM. She correctly guessed that B was a machine-cut nail and C was a wire nail. A was in fact a hand-wrought nail. The telltale sign is the shovel-shaped tip.

Hand-wrought nails are the oldest type. They have been around for thousands of years. Beside a shovel-tip, they can be identified by the fact that they taper in two dimensions.

Cut-nails are made from sheets of metal by a machine. They came into wide spread use in the 1830s. Cut nails have two parallel sides and two tapering sides. They taper in only one dimension because they are cut from a steel plate of relatively uniform thickness. Some cut nails are hand-headed.

Wire nails appeared in the 1880s and supplanted machine-cut nails by 1910, although cut nails are still used for masonry work. Wire nail manufacture is entirely mechanized. This is the most common type of nail in use today.

This weeks artifacts:

Here's a hint: these are not prehistoric or Native American artifacts.


A Note from Florida

Bonnie and I return home today from the annual conference of the Society for Historical Archaeology. April and the Heidelberg Eight (students from the university that she brought to the conference) should be nearly home by now, setting forth on the long drive yesterday evening.

I think we got some interesting feedback on the two that will help us evaluate what we are doing at Port Tobacco.

I thank the GAC crew for soldiering on while I was gone and for their series of blogs about the equipment that we use to recover not only artifacts, but information, from archaeological deposits.

In reference to Anne's blog yesterday about metal detecting, I thought I would mention that we are beginning to experiment with the instrument to identify the locations of buildings on badly eroded sites where little else might remain other than nails and other hardware. I expect we will have some fairly interesting results to report in a few months based on work on a number of sites in Prince George's County.

I look forward to getting back into the trenches.