Below: This photo and the one at the bottom of this page were taken near the Missouri River northeast of Ft. Benton in the White Cliffs area. It won't be too many more
years before erosion will "set free" the iron
concretion shown in the photo
In order to have sandstone the grains of sand must be cemented together. Minerals dissolved in groundwater will do the trick. Calcite (calcium carbonate) is a common cement for sandstone. Apparently what happens to initiate the formation a concretion is that an iron compound starts to be deposited around some sort of nucleus. As the deposition (precipitation) of the iron compound grows around this nucleus, you have the makings of a concretion. This is an area where the sandstone will be tougher because it has been cemented with an iron compound, rather than calcite.
Below: concretions come in all sizes, ranging from the very large ones near the Metra to the bite-sized ones shown below.
These concretions were found on a cliff above the Missouri River south of Big Sandy. CLICK HERE for a closer view.