Turns out these unique fence posts have become a cultural icon of central Kansas. With the Homestead Act of 1862, the lands of the west were opened for settlement. Pioneers were drawn by the lure of relatively free land. Since the prairies are basically treeless, they had to find some way to enclose their new land.
Some smart person decided that the limestone of the area fit the bill for fence posts. It is relatively close to the surface and is pretty uniform in thickness. But the best quality is the fact that it is relatively soft when first quarried but then hardens with exposure to air.
A very cool discovery on the backroads of Kansas!