Mill Creek chert

[2] The chert was used extensively for the production of utilitarian tools such as hoes and spades, and for polished ceremonial objects such as bifaces, spatulate celts and maces.

The nodules were formed as part of the Ullin limestone formation during the Mississippian geologic period (roughly 359 to 318 million years ago).

Spades were chipped from large pieces of tabular flint from sources like Mill Creek, Dover, and Kaolin chert.

The technological leap created by mastering the spade production and the cultivation of maize was one of the single most important events over man's 14,000 year prehistory in America.

At the turn of the 20th century, archaeologists began realizing that in the hilly lands of Southern Illinois was the location for the quarrying and production centers—one of the greatest in prehistoric North America for this type of stone.

[3] From this collection of sites, known colloquially as the "Indian Diggings", Native Americans quarried, worked into tools and blanks, and exported this stone to the wider Mississippian world.

Examples are numerous at Cahokia, where it was especially prized for hoes and spades,[1] but finds have been made in locations as distant as Spiro and Moundville.

[1] The Mississippian culture's heavy dependence on maize agriculture and their monumental architecture (platform mounds, moats and embankments) made such tools especially valuable.