Below: It won't be too many more
years before erosion will "set free" the iron
concretion shown in the photo
below. 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 begin the formation an iron concretion is that iron starts to be deposited around some sort of nucleus within the sandstone. What serves as this nucleus is not well understood. Nonetheless, as the deposition of iron 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 iron oxide, 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.  Check out three photos of a strange concretion (double concretion?) found near Worland, Wyoming. (1.1 Mb - 1.3 Mb) |
