|
They're called
"iron concretions." 
The METRA . . .
This photo
was taken in front of The METRAPARK Arena
in Billings. The "METRA" is a popular venue
for state basketball tournaments, concerts,
rodeos, arena football games, hockey games,
etc. As crews began preparing the site for
construction in the mid-1970s, they came
upon several unusual, large sandstone
spheres, including the ones in the photo.
Rather than breaking them up and hauling
them away, it was decided to landscape
around the huge stones.
Natural Cements . . .
The unique stones are a type of rock that
geologists call "concretions", and their
formation has to do with a variaton in the type
of cement that holds the sandstone together.
In order for sand to become stone, the grains
of sand must be cemented together. Minerals
dissolved in groundwater typically do the trick.
Calcite (calcium carbonate) is the most
common cement for sandstone. But, since
calcite can be dissolved by rainwater,
sandstone is eroded away fairly easily.
However, with the sandstone that concretions
are made of the cement is not calcite, but
rather a more durable iron compound (iron
pyrite or hematite). The mystery . . .
Apparently what
happens to form a concretion is that an iron
compound starts to be deposited around
some sort of nucleus within the sandstone.
What serves as this “nucleus” is not well-understood. Some geologists suggest that it
may be some sort of organism that was
buried in the sand. Nonetheless, as the
deposition of iron grows around this nucleus,
you have the makings of a concretion; a
spherical area within a layer of sandstone
where the sandstone is tougher because it
has been cemented with an iron compound,
rather than calcite. Then, over time, as the
weaker sandstone surrounding the
concretions is eroded away, the spheres
resist erosion as the METRA concretions
did.
Term: weathering
|
