Shell middens, also known as kitchen middens, are archeological sites formed through the accumulation of domestic refuse consisting primarily of mollusc shells. Although large numbers of freshwater shells have occasionally been found in middens in the interior of the Coastal Plain, the great majority of shell middens in Texas are located near the coast and are marked by estuarine shellfish species. Approximately 80 percent of these are the brackish-water clam Rangia cuneata, and most of the remainder are the bay oyster, Crassostrea virginica. A few other estuarine mussels, clams, and snails also occur, as do both marine and freshwater shellfish species. Shell middens occur on the coast of Texas principally from Corpus Christi Bay northeastward to the Louisiana border. Until roughly the 1950s they were also found around Baffin Bay, but these apparently have all been destroyed by erosion of the bay margin. To the east of Texas, shell middens are still found along the Gulf Coast all the way to the Florida Keys. The shells in the Texas middens are the discarded remains of meals prepared by prehistoric and historic Indians. Though some middens elsewhere are the remains of shellfish-drying stations or shell-artifact manufacturing sites, there is no evidence that the Texas middens were formed through any activity other than subsistence. At one time it was suggested that the Texas shell middens were produced by such natural processes as storms along the coast, which washed shells from the bay bottoms up into accumulations on the beaches. However, investigation has shown that the middens contain undisturbed strata, artifacts, and hearth structures and consequently could not have been made by storm-driven wave action.
Because shell middens once were Indian campsites they also contain bones of fish, mammals, reptiles, and other vertebrates and artifacts such as projectile points, potsherds, and other tools. Some shell middens represent the residue from a single meal for perhaps three or four individuals. Others are large mounds more than 500 meters long, 100 meters wide, and perhaps 10 meters thick; they may contain millions of shells. The two oldest known shell middens on the Texas coast are located near Galveston Bay and were initially occupied as Indian campsites about 3,500 years ago. Older shell middens are thought to exist but are now buried under the water and sediment of the coastal bays and the continental shelf. Shell middens do not occur randomly on the coast. They occur in particular locations which, at the time of Indian occupancy, were conducive to shellfish growth. These habitats have shifted location as coastal geography has developed over the past 4,000 years. Consequently, the occurrence of a shell midden records three dimensions of past environments: Indian settlement patterns, past geography, and past natural habitats.
From observation of the condition, form, structure, chemical composition, and population characteristics of mollusc shells in shell middens, conclusions usually may be drawn about the age of the shells and of their archeological context, the season in which collection of the shellfish occurred, the role of molluscs in the native diet, effects of collection on shellfish populations, and reconstruction of the region's paleotemperature record-a major element of documenting ancient climates. In addition, calcium carbonate leached from the shells by groundwater causes alkaline enrichment of the normally acidic coastal soils and makes possible a nearly total preservation of whatever bone material may have been deposited in the archeological site. Moreover, the hard shells tightly wedged together in the midden discourage burrowing animals and limit the churning and disturbing effects of soil desiccation and hydration. For these reasons shell middens are exceedingly valuable not just as a historical and cultural resource, but as a tool for environmental monitoring. Archeological observations of molluscs and other midden materials often are convertible to baseline data about the status of coastal environments before the alteration of global air and water chemistry that began in the nineteenth century. However, despite the value of shell middens, they are disappearing through erosion and use as construction materials. Virtually all shell middens around Galveston and Baffin bays have now disappeared as a result of bay-margin erosion, for instance. Because of this severe attrition, the shell middens that remain are increasingly valuable resources. Most of the investigations of coastal shell middens conducted in Texas have been performed by archeologists from the University of Texas at Austin. The collections and records from these excavations are curated at the Texas Archeological Research Laboratory, University of Texas at Austin.