Shell Midden Analysis

In archaeology, the term shell midden analysis refers to the study of marine shell valves that were once used as food by prehistoric peoples. In the United States, North American Indian tribes who lived near coastal areas often collected clams, oysters, mussel, and other species of shellfish to supplement their diets. Once the meat was extracted, the remaining shells were sometimes used to make ornaments such as beads or carved into fishhooks. However, most of the shell was simply thrown away as waste. It was not uncommon for prehistoric peoples to discard unwanted refuse at centralized trash sites. Over many hundreds of years, shell refuse and soil would build up at these trash sites, resulting in the formation of mounds on what was once level ground. Along the coast of California, for example, shell middens are one of the most distinctive types of archaeological sites. Some of the largest of these middens are over 30 ft (9 m) in depth and may extend more than one-quarter mile (400 m) across.

Once a shell midden has been excavated by archaeologists, the first step in the analysis is to catalog the finds. Typically, the process of cataloguing involves counting the actual number of shell valve specimens that have been recovered. This process includes speciation, determining what species of shells are represented in the collection. Shells are visually inspected, separated according to genera or family, and then subclassified into species. Because certain shellfish species are known to live in specific marine habitats, such as mud flats or open surf, the information gathered from this preliminary study can reveal where and how far prehistoric peoples traveled to gather shells.

Marine shell valves, such as clams, are also studied for their growth rings, which are similar to the growth rings of a tree. These rings or ridges on the outer surface of the shell can yield information regarding the relative age of the animal before it was harvested. Additionally, growth rings can reveal the approximate season of the year when the shellfish was collected. This information is extremely useful to the overall archaeological study, and can be used as evidence in determining whether the campsite associated with the shell midden was inhabited only on a seasonal basis or all year long.

Perhaps the most important analysis conducted on marine shell is radiocarbon or C-14 dating. Often, village and campsites do not produce sufficient quantities of organic material to conduct radiocarbon analyses. However, archaeological sites that have associated shell middens nearby can usually produce more than enough material for extensive radiocarbon studies.

Under controlled scientific excavations and laboratory analysis, shell middens can supply information on marine shell harvesting techniques, trade, subsistence, settlement patterns, and prehistoric environmental conditions. Coupling this data with information from other studies adds to our understanding of the culture and lifestyles of ancient peoples.