by Cherie Haury-Artz, University of Iowa Office of the State Archaeologist
There are a number of naturally occurring geological formations which are easily misinterpreted as cultural artifacts. This is not surprising since their unusual shapes are often so regular that they appear to have been made by humans. OSA often receives images for identification that include the common geologic specimens outlined below. We acknowledge that some items are more culturally "suspicious" than others, so we give strong consideration to context. If found in direct association with obvious archaeological artifacts like spear points, pieces of pottery, metal trade axes, or glass beads, then we would have an archaeological context for them.
These are accumulations of minerals that form around a central object or axis of deposition after a sedimentary deposit has been laid down. They are usually round or disk shaped and may have a hollow center. Sometimes concentric rings of deposition are visible. Less commonly, they can have fantastic and complex forms. Concretions are most often cemented with calcite, dolomite, iron oxide, or silica, making the concretion harder than the sedimentary rock that contains it. When the softer, sedimentary rock weathers away the harder concretion is left behind.
The inside and outside of an iron oxide concretion.
A nodule is an irregular, knobby-surfaced body that is formed from a different mineral than the sedimentary rock that contains it. Nodules are most often formed of silica in the form of chert and are commonly found in limestone or dolomite. Nodules can be round or globular or they may blend together to form continuous beds. When a nodule drops out of a block of limestone it may leave a round depression that can be mistaken for a bowl, grinding stone, or other human artifact.
Chert nodules in situ. Image by R. Weller/Cochise College
Irregular Limestone Formations
Limestone is a sedimentary rock which is formed on the floors of oceans and seas. It is made up chiefly of the mineral calcium carbonate (CaCO3), often from shells and bones of organisms that lived in the sea. Limestone is readily subject to physical and chemical weathering and can erode into many unusual shapes, some may take the form of round bowls or have depressions that look like thumb prints.
Potholes and Round Balls
The power of running water can be quite amazing. A creek or river flowing over bedrock will subject the rock to both physical and chemical weathering, wearing the rock away and rounding sharp edges. The weathering can be irregular with softer rock wearing away more quickly than harder materials. But it is not water alone that wears away at the bedrock in the river bottom. Flowing water carries along other objects including loose stones. The size of the stones being moved is only limited by the force of the flowing water. These stones can get caught in a depression or pit in the bedrock. The moving water continues to bounce them around enlarging the depression and rounding the edges of the stone. This can create depressions that look like bowls, cups, or grinding stones. Geologists call these pits “potholes”. The stone within the depression can also become rounded into an almost perfect ball. Geologists associate potholes with large volumes of very turbulent water. Most often they are thought to be associated with the melt water of a glacier. Both the pot holes and the ball-shaped stones are easily mistaken for human artifacts. It is reasonable to assume that the round stone balls may have been collected and used as toys or games by Native American children of the past so some may have become artifacts, however, they are a natural phenomenon. For an illustration of the process and more discussion see: http://www.geo.mtu.edu/KeweenawGeoheritage/The_Fault/Potholes.html
"Omarulluk" or Omars
These are unusual stones found rocky beaches in Wisconsin and Minnesota. These rounded cobbles have deep pits and hollows that are usually between a half inch and 3 inches in diameter. The holes are created by the erosion of concretions once contained within the stone. The concretions are relicts of cyanobacteria colonies that grew in the water between sand grains in the sediment. For images and detailed discussion of omars see: http://www.geo.mtu.edu/KeweenawGeoheritage/BeachStones/Omars.html