These are lithified sand dunes. These sand dunes once formed the shores of the St. Peter sea, which covered this part of Illinois during the Ordovician Period. These ancient dunes eventually became what is today known as St. Peter Sandstone; a highly pure, medium to fine-grained sandstone consisting of well rounded, almost pure quartz grains. Its origins began in the Precambrian igneous and metamorphic rock of present day Canada, and was carried here by stream as small grains of sand, silt, and shale. Once here, the wave and current action on the shores of the St. Peter Sea worked the deposits into what it is today.