Introduction to Archaeology: Class 10

ã Copyright Bruce Owen 2002

• Thomas jumps right from finding sites (site survey) to using high-tech remote sensing and then digging.
• This leaves out two essential steps that are very frequently used
• much cheaper and often easier (at least the field part)
• very informative
• that is, mapping the site, and then making and analyzing surface collections
• Mapping
• A whole range of methods, from by-eye sketch diagrams to high-tech 3D modeling
• Pace-and-compass, or tape-and-compass mapping
• measure your stride so that counted paces can be converted to meters or feet
• or use long measuring tapes (50 meter or 100 meter are typical)
• record angles and lengths of line segments between major "control" points in closed loops
• control points are fixed features of the site, like the tip of a big rock, the corner of a building, etc.
• closing allows you to see how far off the measurements are and distribute an adjustment around all the angles equally
• then measure in additional features relative to the control points
• Grid mapping with long tape measures or theodolite
• another approach is to lay out a rectangular grid across the site, and map in features relative to the grid points
• the grid can be laid out using long tapes and a few right angles
• right angles can be determined by compass
• or by measuring right triangles (using known ratios of lengths of right triangles, like 3:4:5, or 1:1:1.414
• the grid can also be laid out using a theodolite: a small telescope mounted on a tripod with very accurate scales that let you measure horizontal and vertical angles
• theodolites are often used with stadia rods: essentially a very long ruler that is held on a given spot
• the farther away the stadia rod is, the more of the "ruler marks" appear in the view of the theodolite
• this can be used to calculate how far away the stadia rod is, without having to stretch a tape to every point
• Standard mapping with a theodolite and stadia rod
• set up the theodolite in one, central spot
• have the "rod man" stand the stadia rod on each point of interest
• measure the distance to it using the stadia markings
• measure the horizontal angle using the angle ring on the theodolite
• record these two values and/or directly plot them on a map
• Plane table mapping
• same idea as standard theodolite and stadia rod mapping, but the telescope sits on a flat, rectangular plate, resting on a flat table with paper attached to it and a small pin sticking up in the center, through the paper
• point the telescope at the stadia rod standing on the point of interest
• rest the edge of the flat plate against the pin
• draw a line along the edge of the plate; it points straight to the stadia rod
• measure the distance using the stadia markings
• multiply that measurement by the map scale, then measure along the line on the paper to directly plot the point.
• the map is drawn directly on the paper, point by point.
• Total station, laser rangefinder, etc. mapping
• instead of a stadia rod to measure distance, use a laser mounted on the telescope and a reflector that the "rod man" holds on the point of interest to measure the distance
• some even can reflect the laser off any surface without using a reflector, largely eliminating the "rod man"
• the total station instrument also measures the horizontal and vertical angles very accurately
• often converts the measurements to x, y, z coordinates of the point
• often outputs directly to computer files or to a mapping program
• same logic as a standard theodolite map, but much, much more accurate and much faster
• but the instruments are large, heavy and cost five to ten thousand dollars and up
• GPS mapping (Global Positioning System)
• for medium-precision mapping of site boundaries, general outlines of large structures like Maya temples, canals, roads, etc.
• GPS alone is about ±10 meters
• good enough for locating structures or outlining very large ones
• often using a "base station" to give greater precision
• accurate to a fraction of a meter, say 50 or 75 cm.
• good enough for outlining structures, but not for mapping their details
• (versus a total station map, which should be accurate to a few centimeters or even better)
• 3-D scanning
• the latest method, *very* expensive but incredible for many purposes
• set up a machine on a tripod
• it scans the whole area in front of it with a laser beam, noting the angle of the beam and the distance to the object that it strikes
• feeds this data to a computer modeling program
• that instantly builds up a 3-D model of the surfaces in front of it
• gives an extremely accurate, 3-D model of the ground, walls, stones, etc. that you can rotate, zoom, measure, etc.
• since walls will block parts of the view, a number of setups are required to look at the site from different angles, and these are merged by the computer
• a large, complex site can be fully mapped in a couple of days, instead of weeks, and with much greater precision and detail
• great for sculptures, sites with standing architecture
• useless for sherd scatters, patches of midden (garbage), etc. where the features to be mapped are not 3-dimensional, but depend on identifying small objects, soil color, etc.
• developed for documenting rebar, plumbing, etc. on construction sites before it is encased forever in cement, for legal purposes
• machine costs in the six figures
• What can we learn from site maps?
• how big the site was
• features like walls, flat spots, patches of garbage, etc. may indicate where households were, and give some idea of the density and population of the site
• other features may suggest the patterning of activities; maybe the housing was in one area, cooking areas in another, and cemetery in yet another
• architecture can suggest all sorts of things: was the site residential? used for storage? monumental? was access restricted, or open? was it defensible? was there an open plaza, variation in the size and complexity of buildings, etc.
• the map will then guide further work (if any), like surface collections, non-invasive remote sensing, and excavation
• the data from all of these methods is almost always best understood by placing it in context on the site map
• Surface collections
• Once the map is made, it is usually helpful to know what sort of materials are found on the surface of different parts of the site
• this can tell us about differences in activities in different areas
• lots of blackened sherds of plain cooking pots probably indicate places where people lived
• stone tools for grinding grain would also suggest that
• flakes from making stone tools might indicate workshop areas
• fancy, decorated sherds might indicate higher-status residences, or areas that were more public, where display of wealth was important
• Almost always, only a sample of the material on the surface is collected
• picking up everything would not tell you much more, and would require a lot more effort to analyze and store
• The same sampling strategy issues that we discussed for survey also apply to deciding specifically where to make surface collections
• judgment sampling, systematic, random, stratified random, transects...
• surface collections are often done in round "dog-leash" units, sometimes in rectangles, etc.; whatever is convenient and provides systematic samples that are large enough to be useful
• collections may include only ceramics, or may also include other things such as lithics, shell, etc., depending on the issues to be addressed
• the objects in each collection are then categorized using a typology, recorded, and eventually plotted on the site map to see if there is any interesting patterning
• there are many ways to do this
• La Cantera examples
• Limitations to mapping and surface collection
• these methods work best on sites that have "high surface visibility"
• that is, they are not deeply buried
• nor overgrown with vegetation
• they also work best on sites that have few components, especially single-component sites
• on multi-component sites, the surface material will be a mixture of stuff from more than one time period
• so it can be hard to separate out a picture of any one period of occupation
• like a series of maps on overhead transparencies stacked on top of each other
• on the other hand, the surface collections may help to separate out the different components that are mixed together on the map
• for example, if the ceramics from one period are all found in one sector of a site, and the ceramics from another period are found in a different sector, then you may be able to separate the map into two, smaller maps that each refer to a different component or time period
• any areas of overlap may also show up as having both kinds of ceramics
• these might be good places to dig to find superimposed strata of the two components
• Remote sensing or noninvasive methods
• Aerial photography
• like what we saw for survey, but used for detailed views of sites
• much like mapping, but only shows things that are visible from high up (walls, not scatters of lithic flakes)
• may show features of relief, ground texture, color, etc. that are not obvious from the ground
• with proper controls, may be used directly for mapping
• Physical methods that detect anomalies in the soil below the surface
• these depend on there being features in the soil that have different physical properties the rest of the soil
• voids (open spaces)
• rocks, if the soil is not rocky, especially large ones or arrangements like rows or walls
• areas the hold more or less water than others, like looser fill in wall trenches
• areas that have been heated by fire vs. the rest of the soil that has not
• and others....
• different methods respond to different kinds of contrasts in the soil; it is often hard to predict which, if any, method will work well on a given site
• and some sites just are unlikely to yield anything
• sites without much massive architecture or many features dug into the ground, like foragers' camps or perishable housing made of cane, wood, etc.
• sites in which the architecture is not very different from the soil
• like thin or low walls made from the same rocks that the soil is full of
• Soil resistivity mapping
• uses a system of metal probes stuck into the ground to measure how well the soil conducts electricity
• the spacing of the probes adjusts the depth of the soil included in the measurement, in a very rough way
• particularly good for differences in water content, like rocks vs. soil or looser trench fill vs. denser natural subsoil
• gives very vague patterns, but linear features or big anomalies may show up
• Ground-penetrating radar
• covered by Thomas
• sensitive to various different kinds of anomalies, not easy to interpret
• Magnetometry (typically with a proton magnetometer or a similar instrument)
• sensitive to differences in the magnetic qualities of soil and rocks
• works best in places where rocks from one area have been brought into a geologically different area, as in basalt Olmec sculptures in a limestone area
• or where large patches of certain types of soil have been heated, as in locating buried kilns or ceramic firing spots
• in all these methods, the data are plotted on the site map, and one looks for
• linear features that might be walls or trenches
• patches of different values that might be pits, large rocks, or voids like tomb chambers
• any sorts of anomalies that seem to relate to features visible on the surface, like areas with certain kinds of sherds or within particular areas of the architecture
• interpreting the anomalies is rarely straightforward
• usually, you have to dig in a few places to learn what causes them in this particular site
• but once you have that worked out, the patterns may allow you to trace walls, trenches, etc. over broad areas without having to excavate them.