In 1998, five drains were installed into the west slump of Monks Mound in a fan-shaped pattern from the base of the mound. While boring for the fifth drain, the drill operator was into the mound about 150 feet and about 45 feet below the surface of the Second Terrace when he sensed that the rig was hitting stones.

He continued drilling and went through 32 feet of stones, which, based on his experience, he said felt like soft stone (such as limestone or sandstone) cobbles, maybe up to a foot in diameter. As he attempted to retract the drill, the bit broke off in the stone mass, so now there is a $5000 bit with an electronic sensor that has become a new artifact in Monks Mound! There was no attempt to continue drilling to install this drain. Unfortunately, the drain system has had only limited success and does not seem to be moving the amount of water we would like.

Thus, it is now known there is a mass of stones that extends at least 32 feet in one direction, but we have no idea how thick it is or how much area it covers, or what it represents. It is unique to Mississippian sites as there are few uses of stone for any type of construction that have been documented, other than a few stone slab graves in some areas. Even where stone was available, Mississippians preferred earth and wood for building materials.

Obviously, there is much speculation as to what it might be. A tomb? A ceremonial platform? A building or structure of some kind? Just a pile of rocks? An attempt at erosion repairs? The function will be difficult to determine and the exact nature of the stones may never be known. The Mississippians had to go some distance to get stone, as it does not occur naturally in the floodplain. There are outcrops of limestone and sandstone along the bluffs 10-15 miles away near Alton and Dupo, Illinois, as well as about 8 miles to the west on the St. Louis side of the river. Thus, it had to be transported a great distance, which indicates that it must have had some significant or ceremonial purpose to make that great an effort to get it here. It is doubtful it is an erosion repair since those could have been more easily repaired with driven wood pilings or wooden "dams" in gullies to prevent further erosion. Besides, they had thousands of people available to hand-carry baskets of soil back up the slopes to patch and tamp down in areas that failed.

From vertical corings that have been done through Monks Mound from the 1960s to the 1990s, archaeologists know that the stone is not under the entire mound, it just seems to be concentrated in this area of the west side. Some archaeologists made an attempt with hand bucket augurs to core down to the stones, but were only able to penetrate about 33 feet down, where they encountered an extremely hard layer of blackish clay with a lot of matted vegetation on it. This may represent a surface of small mound built over the stones or an earlier surface of the whole mound and it may be this dense clay layer that prevents the internal water from going deeper, creating ponding, or a perched water table within the mound. In fact, when the auguring was done the hole filled very rapidly with water, almost like it was under pressure.

Another attempt was made with a drilling rig that was on loan for a half-day from another archaeological project. They drilled down about 43 feet and had not yet hit the stones when they had to stop and get back to Wisconsin. However, when examined later, the deepest soils in the core had small flecks and fragments of what appeared to be limestone, so the coring may have just approached the surface of the stone mass.

Will we ever know more about these stones?
The stones are too deep to dig down to with conventional excavations, as one would have to dig away a large section of the west side of Monks Mound to get that deep safely, and that would have too much impact on the mound. There are a number of remote sensing tests using a variety of instruments to "sense" what is below the surface. Crews have tried using magnetic and electronic and radar techniques, but none of them can penetrate deep enough to provide significant clues. In the future, if funding becomes available, we hope to try some "seismic sounding," which should penetrate to that depth and at least tell us how big an area is covered by the stone mass and perhaps its shape to some degree. However, it will not tell us exactly what it is, so that may forever remain a mystery, but we can always continue to speculate until some new technology is developed that will help us "see" into the mound.