A publication of the Archaeological Institute of America
Contractors drilling into the western side of Monks Mound at the Cahokia site in Illinois have discovered a layer of limestone or sandstone cobbles deep within it. The drilling was part of an effort, being monitored by archaeologist William Woods of Southern Illinois University at Edwardsville, to stabilize the mound following the slumping of earth from its sides, most recently after heavy rains in 1993 and 1994. Field directors James Burns, Andy Martignoni, Jr., and Steve Fulton were surprised by the layer because no stone is available at the site and conventional wisdom has it that the Mississippian Indians who built the mound used only earth.
The principal attraction at Cahokia Mounds State Historic Site, on the Mississippi floodplain across the river from St. Louis, Monks Mound was begun about A.D. 950 and largely completed by about 1150, although additions were made up to 1250 (see "Mighty Cahokia," May/June 1996). It measures about 1,000 feet north-south by 770 feet east-west and rises more than 100 feet. The stone layer, found 140 feet into the mound and 40 feet beneath the surface of the second terrace, extends for at least 32 feet. How large an area the stones cover and how deep they reach are unknown.
William Iseminger, a curator at Cahokia, believes the stone layer could be some kind of ritual platform and notes that stone had to be brought from ten to 15 miles away. Because of its depth, it will probably never be excavated, an effort that might harm the mound's structural integrity. Plans have been made to conduct seismic and resistivity surveys this July. In addition, small holes will be drilled vertically through the mound to extract rock and soil samples.
The mound, which has been closed during installation of new steps, is to reopen to the public late this spring.