Sunday, June 16, 2013

Day 10: Stratigraphy, Plan Drawings, and the Harris Matrix

Day 10- Stratigraphy, Plan Drawings, and the Harris Matrix

There are a lot of different terms when it comes to stratigraphy, so here we go! Stratigraphy is the process of recording and analysis of site location. Stratification is the physical layering of deposits on a site. Stratigraphy is governed by three principles (for sedimentary deposits), the law of super position, is the order in which sediments are laid down, the principle of continuity, and the principle of original horizontality, ruling that layers will continue in the same pattern of layering over geological breaks (i.e. earthquakes, etc.). It would look like:

Soil A│(break in the earth)│Soil B

Deposits are defined as the assembling and layering down sediment and its inclusions, they can be divided into single events and episodes of deposits. Context (or stratum) is defined by an excavator, may represent a single deposit, may also be arbitrarily defined (since the excavator defines it), and can be known as stratigraphic units. An interface is a boundary between two or more deposits, the soil color, inclusions, or texture change as seen in quadrats (Horizon A, Horizon B), can be seen laid between the top soil and the sub-soil in some instances. If a relationship is found between two non-contiguous contexts, here at Monticello, we refer to it as a correlation, this involves a level of interpretation, and relating vertical texture to past excavations for reference. As we dig through the various matrices of sediments, we often come across non-movable artifacts (non-portable, i.e. boulder or sub-floor pits), these are referred to as features in the context records and archaeological jargon.

As with anything in archaeology, there are types of stratification, used as various ways of defining the horizons. Lithostratigraphy defines horizons through geological/pedological principles, is based on the natural soil formation processes, and applies to all horizons. Biostratigraphy defines the horizon based on presence/absences of life forms, such as fossils, and pollen analysis. Ethnostratigraphy is defined through evidence of cultural activities, such as plowing, or chemical testing for manure use in farming. All of these various forms of stratification are all not mutually exclusive and all of these are used by the archaeologist to date a site (i.e. not just ceramics).

It is the job of the archaeologist then to map these horizons, as the very act of excavation is destruction, it is important to record every detail, through each step of the dig. The goal of mapping the site is to record and show the natural and arbitrary boundaries of the excavation and to differentiate between the two types of interfaces. Quadrats are the arbitrary boundaries and use the mapping notation of line-dot-dash, natural boundaries (cliff, river, or sediment change) are drawn with solid lines to represent separate contexts. These mapping devices are used to illustrate the physical relationships of the site, as well as the stratigraphic relationships. An example of this vigorous record attention can be found in the field, as whenever we close a context in our quadrats, we have to map and record (known as quadrat drawings) the geometric location of each feature (rocks mostly), in order to record each step of the land we dig out, so that future archaeologists can refer to our notes and drawings in the future if need be.

There are various models to follow when plan drawing, there is the single-level plan (“top” plan), the phase plan drawing, and the single context plan. The “top” plan has no discrete boundaries, mostly used in large open excavations, and requires the mapping of each day’s work. The phase plan is more interpretive, placing all things together on one map which are thought to be from the same time period, this model obviously has a more chronological focus. The single context plan is what archaeologists of Monticello use, focusing on each context as a separate quantity, including the natural interfaces between a context and their relationship physically to one another. In order to pull of these descriptions together from the 266 different quadrats dug at Monticello, a device known as a Harris Matrix is used to date the multiple stratigraphic layers, and also to show the chronological sequence of the several dig sites on the mountain. The Harris Matrix represents a form of reverse engineering, structuring the sediments (including features), in conjunction with the artifacts found within the quadrats, and then placing that structure into a chronological chain, with the youngest top soil on top (labeled “A”) and the oldest on the bottom link of the chain, etc. This is an easy way to collect data from a hundred-some plan drawings, descriptions, and artifact finds into one easily interpreted chart. The Harris Matrix is always defines a temporal relationship and is not a fine scale. (below- an example of a Harris Matrix, not relating to Monticello)

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