The oldest stone artifacts found are at least 2 million years old. In contrast, some of the oldest organic remains found are woven Fort-Rock-type sandals from Elephant Mountain Cave in southwestern Oregon that date to approximately 10,000 years ago. That is quite a difference! Why are some artifacts preserved so much longer than others?
Though stone can weather, it is inherently stable compared to organic materials used for basketry and tools. Artifacts made of wood, papyrus, and bone, for example, are perishable and prone to rapid decomposition (rot and decay), making them difficult to recover. Decomposition occurs when organisms such as molds and bacteria inhabit and often consume organic material. Those organisms require warmth, water, and oxygen to survive. Without those components, the environment is sterile and decomposition will not occur, greatly increasing the potential for organic artifacts to be preserved. Therefore, a sterile environment that is free from microorganisms provides the best conditions for preservation of a wide variety of archaeological remains.
Extremely cold environments such as glaciers and year-round snowpack also provide optimal conditions for preservation. A bittersweet benefit of climate change is that many archaeological sites are being uncovered by the thawing of glaciers around the world. Examples of finds from such ice patch archaeology include atlatl (spear thrower) darts, baskets, clothing, and arrows still coated in paint and decorated with feathers. Other things that have been preserved in cold conditions include entire bodies of animals such as mammoths and even humans (Ötzi the ice man is perhaps the most famous example) that were frozen soon after their deaths and left unchanged until being recently uncovered.
Oxygen is necessary for the microorganisms that cause decay to survive and reproduce so remains that were enveloped in sealed, airless environments—typically waterlogged conditions such as bogs—can be remarkably well-preserved. Bogs commonly produce oxygen-deprived anaerobic conditions. Examples of items that archaeologists have recovered from bogs include carts, wooden roads, and even ships such as ones from the 1800s and 1900s found in areas that were formerly part of San Francisco’s waterfront. One exceptional find is the nearly 50 “bog bodies” discovered at sites around the world, including the Windover bog body from Florida (researchers were able to identify his last meal) and Tollund Man from Denmark. These human remains are remarkably well-preserved thanks to conditions in the bogs. Their skin is darkened but is preserved, as are many internal organs, while their bones typically are not preserved.
The third condition necessary for decay is water. The extremely dry conditions typically found in deserts thus allow for preservation of textiles, basketry, and other woven items. Entire Navajo hogans (homes) have been preserved in the American Southwest. Perhaps most remarkable is the natural mummification of bodies in places such as Peru and Egypt. Preservation can be so complete that Egyptian mummies retain their fingernails and hair and Incan mummies still have visible tattoos on their skin and retain the woven textiles in which their bodies were wrapped after death.
The problem with artifacts recovered from these three types of conditions is that artifacts remain well-preserved only as long as they’re kept in the conditions that allowed for their preservation. Once ancient wood is removed from waterlogged conditions, it splinters, cracks, and quickly begins to decompose. Likewise, once mummified bodies are exposed to normal (moist rather than dry) conditions, decomposition resumes. Once all three of the elements required for microorganisms to grow—heat, moisture, and oxygen—are restored, decomposition will occur and can actually be accelerated, sometimes quite dramatically. The terra cotta warriors in Xi’an, China, for example, had been preserved for more than 2,000 years in an enclosed chamber underground. Since being exposed in 1979, the 7,000 unique, brightly painted terra cotta warrior figures’ colors have faded, and the figures themselves have begun to disintegrate. Excavations at the site were halted until researchers could find a way to prevent further damage.
The materials from which archaeological remains were made also affect their preservation, or lack of it. Objects made of stone, clay, and metal are commonly found at archaeological sites. They are the most commonly preserved materials because they are inorganic and do not decompose, though they can weather, a process in which wind and water erode and wear away their surfaces. Stone tools dating back more than 2.5 million years have been found in archaeological sites and many are so well preserved that microscopic wear analysis can determine how the stones were used and even the dominant hand of the tool maker. Baked clay and pottery can also survive most conditions as long as the items were adequately fired. However, highly acidic soils can damage the surfaces of baked clay items, and objects that were poorly fired or made from porous clay can become quite fragile in humid conditions. Metallic objects made with gold, silver, and lead can survive quite well, but objects made with copper, bronze, and other soft metals usually oxidize (rust) and can completely disintegrate, leaving only a green stain where they once rested. Underwater, metals at sites such as shipwrecks are preserved by the naturally occurring metallic salt in sea water that accumulates on their surfaces. In fact, these salts actually permeate metal objects, and it can be quite laborious to remove the encrustations.
Organic skeletal, animal, and plant remains and textiles all suffer from decomposition, and their survival depends largely on the material surrounding them, called the matrix, and on the climate. In terms of the matrix, chalk tends to preserve bones quite well because it acts as a desiccant, removing all the moisture from objects within it. Acidic soils, on the other hand, tend to destroy bones and wooden artifacts quite quickly. Other conditions of the matrix that can forestall decomposition and promote preservation include natural salt and oil pools that kill bacteria. A famous example in California is the La Brea Tar Pits located in central Los Angeles. This naturally occurring asphalt pool has preserved more than 35 million specimens of plants and animals from the last 50,000 years!
Climate affects preservation of organic remains by dictating how much oxygen, heat, and water are present. Caves are natural conservatories. Their chambers are protected from the elements and the soils and water inside them are often alkaline, which prevents the growth of bacteria, thus protecting whatever is inside them, including footprints! Tropical environments like those found in the Yucatan peninsula where the Maya lived, on the other hand, are typically highly destructive because of their heavy rains, acidic soils, warm temperatures, high humidity, erosion from weather, and abundant plant, animal, and insect activity. Additionally, the overgrowth common in jungles can quickly overwhelm sites, hiding them from outsiders (a good thing for overall preservation since it makes the sites hard for looters to find). The Temple of the Inscriptions at Palenque in Mexico, for example, was once brightly painted, but because of rain, humidity, and heat, the paint on the temple is no more. Temperate climates like those found in North America and Europe also are poor preservers because they are relatively warm but go through seasonal changes in temperature and moisture. One surprisingly good environment for preserving organic artifacts comes from natural disasters. The volcanic eruption of Mt. Vesuvius rapidly covered Pompeii in Greece in ash, and a violent wind storm at Skara Brae in Scotland completely covered the site in sand. Both sites were exquisitely preserved. Rapid burial of sites through flooding, storms, and volcanic eruptions have preserved many of the world’s most impressive archaeological sites.
Terms You Should Know
- anaerobic
- atlatl
- decomposition
- hogan
- ice patch archaeology
- sterile environment
Study Questions
- What three elements are needed for decomposition to occur? What happens when one of those elements is missing?
- Select one of the preservation conditions—cold, dry, or wet—and describe what happens to archaeological materials in that condition.
- Characterize an environment that would generally fail to preserve organic artifacts. Describe at least three characteristics of that environment that thwart preservation.
- Describe an environment in which preservation conditions would be ideal or would likely preserve organic artifacts well. Are there any types of organic artifacts that wouldn’t survive well in that environment?
