Possible impact spherules from near the base of the Middle Ordovician in northern Illinois

Angular fragments of a greyish-white chertlike rock containing numerous sand-size spherules believed to be of impact origin have been found in northern Illinois. These fragments, many as large as a man's fist, are recognizable in the field by the presence in them of conspicuous rounded cavities which were probably bubbles. Most have been picked up as float in the bottoms of ravines, but it is clear that they come from near the base of the St. Peter sandstone and that they were already fragments when incorporated in this formation.