A mound is the generic word for a type of earthwork found throughout the world. Whether they are called by a different name, such as barrow, tumulus, platform mound, rammed earthwork, even some earthen pyramids, mounds are works primarily built of soil, perhaps augmented by a stone or wood foundation. Some are small and barely noticeable rises on the landscape, particularly after the affects of hundreds or thousands of years of erosion. Others can clearly be classed as monumental architecture, requiring the collaborative work of many individuals working for many weeks, months or years. These structures may or may not contain human burials.
The term is most frequently used to describe the Mississippian mounds of the central and eastern North American continent, but the tradition of raising earthworks may be found in one form or another in prehistoric cultures around the world.
Building a Mound
Recent geophysical survey work on Mississippian mounds in the United States has revealed much detail about mound construction methodologies. What has been considered a fairly straightforward process (and often pictured in dioramas) of conscripted people carrying basket loads of soil to build up a mound of earth is turning out to be architecturally and ritually a far more complex process.
First, a location is prepared. Depending on where a mound would be placed, terraforming activities to create the initial footprint might include leveling the region by cutting and filling. Then, the construction material must be located and transported to the site. Materials are usually selected from nearby, but depending on physical requirements and local resources, some mounds are built with soils carried from as far away as 100 meters (~300 feet). Some mounds may be built from multiple sources: some (like Monk's Mound in Cahokia) were built of purposefully mixed soils, perhaps for a ritual or geotechnical purpose.
Mound Construction Types
Mound construction, as defined for the Mississippian examples studied by Sherwood and Kidder, falls into five types: sod blocks, soil blocks, loaded fill, zoned fill and veneers.
- Sod blocks: intact sections of surface soil or turfs cut down through several soil horizons (A, B and/or E horizons). This method has been identified in northern and western European Neolithic and Bronze Age burial mounds, and found at the Mississippian site of Shiloh and Monks Mound at Cahokia. Most often placed with the sod top down.
- Soil blocks: intact sections of soil without the surfaces, consisting of clay-rich B-horizon or C-horizon soils. These were selected by the builders at Shiloh to create a solid internal core.
- Loaded fill: basket-loads of loose soil used to construct one fill event. Massive loaded fills have been identified at Cahokia mounds, consisting of alluvial soils brought in from the nearby floodplain. Loaded fills are often used to reconfigure a mound, to cover a previous construction or to raise the level of an existing mound.
- Zoned fill: purposefully placed light and dark layers, sometimes called "blanket mantle". Alternating types of fill may have had a structural purpose: for example, alternating a water-permeable soil layer with a non-permeable layer might help control erosion. There might also have been a ritual use: in some cases, such as Monks Mound at Cahokia, the layering is visually striking.
- Prepared veneer: thin layers of different source materials placed on the external slopes or stepped surfaces of a mound. Thickness ranges between 2-15 centimeters, depending on the scale of the mound. Veneers have been identified at Cahokia, Shiloh, and Lake George.
Sherwood and Kidder found that of the mounds they examined in detail (Poverty Point, Raffman, Cahokia and Shiloh), construction occurred rapidly, on the order of weeks or months of effort rather than years. The larger mounds (Monks Mound at Cahokia, and Shiloh's Mound A) appeared to have been built and rebuilt over generations.
Sources and Further Information
Sherwood SC, and Kidder TR. 2011. The DaVincis of dirt: Geoarchaeological perspectives on Native American mound building in the Mississippi River basin. Journal of Anthropological Archaeology 30(1):69-87.
Trubitt MBD. 2000. Mound Building and Prestige Goods Exchange: Changing Strategies in the Cahokia Chiefdom. American Antiquity 65(4):669-690.
Van Nest J, Charles DK, Buikstra JE, and Asch DL. 2001. Sod blocks in Illinois Hopewell mounds. American Antiquity 66(4):633-650.