why do archaeologists dig square holes?

 Goal: To understand how archaeologists establish and maintain the context of artifacts. Objectives: Students will be able to: interpret a plan view of an archaeological site locate the position of each excavation unit reconstruct the context of each artifact within the grid explain the importance of the grid system Key Words: Context - the situation or condition in which a site, artifact, or feature is found. Grid unit - a square area of the grid system that is labeled by the coordinates in one corner. Plan view - a map looking down on a site. Provenience - the precise location of an artifact or other archaeological feature based on its position in the grid and how far under the ground surface it is found. Site datum - an arbitrary point established off-site that is used to establish the grid system. It has grid coordinates of (0,0). Background: Click on this picture for a larger version! When a site is excavated or disturbed in any way, artifacts are removed from their original context. Once they are dug up they can never be put back exactly as they were found, so archaeologists must record their position on paper. One way to do this is to establish a grid system over a site. Before excavating, a datum point is established at an arbitrary location and labeled (0,0). From this point a grid of equal-sized squares, or units, is laid out over the site. The corners of each grid unit are labeled according to their distance north, south, east, or west from the site datum. The material excavated from each grid unit is kept separately from the material from other units. In this way, an artifact from one grid unit will have a different provenience than an artifact from another unit. A grid allows archaeologists to reconstruct the location of the artifacts on a map, or plan view, of the site. If the place where each artifact was found can be seen in a plan view, some patterns of artifact distribution may appear. These patterns give archaeologists insights into what activities occurred in a certain part of the site. For instance, shown here is the plan view of a structure at Old Mobile. Notice that most of the brick was found in one area of the site. Archaeologists believe this may be near where the structure's fireplace was located. Old Mobile Activity: Reconstructing the Past Click on this picture for a larger map of Structure X! Materials: Each student should receive a copy of this artifact key and site map of Structure X. Procedure: Explain to the students that a grid has been laid out over the possible site of a building at Old Mobile. As the site is excavated, a list is kept of what artifacts come from each unit. Have students place the letter representing each artifact in the correct unit on the map. Discussion: Artist's reconstruction of a house at Old Mobile based on recent excavations (drawing  by Professor Phillipe Oszuscik, University of South Alabama). The houses at Old Mobile were made mostly of wood and clay. Some houses had a small brick fireplace to provide warmth and a place to cook. Small glass windows might have been found in a few houses. The colonists didn't have trash cans like we do today, so they usually just swept their trash out a door or threw it into a shallow pit that was dug conveniently within a few feet of their house. Many years later, archaeologists have been able to piece together this picture of colonial life by using the grid system. When the location of every artifact is recorded according to the grid unit in which it was found, some patterns of distribution will appear. For instance, a line of iron nails found across several units could indicate that a wall once stood in that area. A cluster of bricks may have been the site of a fireplace. By studying their plan views of artifacts at Structure X, students should be able to answer the following questions. If a concentration of nails indicates that a wooden wall was once present, how many walls of Structure X have been uncovered so far? The location of iron nails in generally only three rows of the grid indicates that only three walls of the structure have been excavated. One corner of the building was probably in grid unit (1,2) What can we tell about the size of Structure X? The southernmost wall of the structure is the only one whose length is fully uncovered, but it can tell us about how wide the building was. According to the scale on the grid unit map, each side of a unit equals 1 meter, so Structure X was around 4 meters wide (about 13.5 feet). What could the concentration of window glass mean? The most likely answer is that a window was once located in the area where the pieces of glass were found. There are two large concentrations of artifacts. The one located outside the walls of Structure X doesn't include brick or fragments of copper kettle. What does this tell us about each of these groups of artifacts? The area of the site that has brick and pieces of copper kettle in addition to animal bone and pottery was probably the location of a fireplace. Found together, these artifacts tell us that cooking activity was taking place here. The concentration of artifacts located outside the structure probably represents a trash pit.