Mounds are elevated earthen structures that appear as small hills, applying to both natural landforms and human-made constructions across the globe. In North America, the most prominent examples are the thousands of prehistoric earthworks built by various Indigenous cultures over a 5,000-year period, from the Archaic through the Mississippian period. These monumental constructions are concentrated throughout the eastern United States, particularly in the Mississippi and Ohio River valleys. They represent the continent’s only significant pre-Columbian monumental architecture outside of Mesoamerica and served various purposes, including burial, ceremonial, and residential functions. Studying these earthworks reveals complex social organization and engineering skill.
Defining the Earthen Structure
Archaeologists must first differentiate between naturally occurring landforms and intentional, human-built artificial mounds. Natural elevations, such as glacial drumlins, are composed of poorly sorted glacial till and are shaped by the movement of ice sheets. This results in an elongated, asymmetrical form aligned with the ancient ice flow direction. These geological features contain sediments that are generally uniform and lack evidence of human selection or placement.
Artificial mounds, by contrast, are structures intentionally built by heaping soil, rock, or other materials onto a natural surface. The physical evidence of their construction is visible through archaeological excavation, which reveals distinct layers of different soil types, clays, and sediments. This layered structure, often seen as “basket-loads” of earth, is a clear indicator that the mound was built by human effort rather than geological processes.
The inclusion of cultural artifacts is another definitive marker of an artificial mound. Fragments of pottery, charcoal, shell, or even evidence of burned structures are often found within the earthen fill, confirming human activity during the construction process. Furthermore, the location of human-made mounds frequently breaks the pattern of the surrounding natural topography, such as being placed in a flat river bottom or forming a geometrically precise complex. These structures are built according to a cultural plan, whereas natural formations are aligned with geological forces and display a more random distribution.
Classifying Mounds by Function
Artificial mounds are broadly categorized based on their intended cultural use, reflecting the diverse needs and beliefs of the societies that constructed them.
One of the most widespread types is the Platform Mound, characterized by a flat top and a truncated, often pyramidal shape. These mounds functioned as substructures for important buildings, such as temples, council houses, or the residences of elite leaders, elevating these structures above the surrounding community.
Platform mounds are strongly associated with the Mississippian culture, particularly at sites like Cahokia, where the immense Monks Mound served as a foundational structure for a large building. The flat summits provided a stage for public rituals and a visual symbol of the political and religious hierarchy of the society. Their shape allowed for multiple stages of construction over time, with new layers of earth added to expand or rebuild the structure.
A second major type is the Burial Mound, which typically has a conical or dome shape and was used for the interment of the dead. Cultures like the Adena and Hopewell in the Ohio Valley are well-known for their elaborate burial mounds. These often contained the remains of individuals along with grave goods like copper ornaments, shell beads, and ceremonial pottery. These mounds were the focal point of complex mortuary rituals, sometimes involving the successive burial of multiple individuals or groups over many generations.
The third significant category is the Effigy Mound, which is shaped like an animal, human figure, or geometric symbol. These mounds, primarily found in the Upper Midwest, particularly Wisconsin, are not always used for burial; their primary function is often ceremonial or symbolic. Effigy mounds are typically low and elongated, depicting cosmologically significant animals such as birds, bears, or serpents, like the famous Serpent Mound in Ohio. Their forms are thought to represent clan totems, territorial markers, or connections to spiritual forces.
Methods of Construction and Cultural Builders
The construction of monumental earthworks required immense, coordinated labor and a deep understanding of soil mechanics. Builders utilized locally available materials, primarily earth, clay, and sand, but also incorporated shell, ash, and sometimes stone. These raw materials were carefully selected and transported from nearby “borrow pits” or surrounding areas to the construction site.
The primary method of moving the soil involved manual labor using simple tools like digging sticks, clamshell hoes, and animal scapulae. The earth was then transported to the mound site in woven baskets. This technique meant that millions of basket-loads of earth were carried and dumped, layer by layer, sometimes over centuries, to achieve the final size and shape of the structure. The distinct, overlapping layers of soil are visible in the mounds’ cross-sections.
These extensive building projects demonstrate the ability of pre-Columbian societies to organize and mobilize large labor forces. The earliest known mound complex, Watson Brake in Louisiana, was built around 3500 BCE by hunter-gatherer societies, showing that mound building was not exclusive to settled agricultural communities. Later cultural builders included the Hopewell and Adena cultures, who focused on conical and geometric earthworks during the Woodland period. The most prolific builders were the Mississippian cultures, who constructed the large flat-topped platform mounds, such as those at the Cahokia site, which thrived between 1050 and 1350 CE, showcasing a peak of complexity in engineering and social organization.