A concretion is an aggregate of inorganic matter in the shape, roughly,
of a ball, disc, rod, or irregular nodular body. Usually the aggregation
or accumulation started around a small center grain or particle and
continued in the growth of layers about it like the shells of an onion,
or in the growth of needle-like fibers which radiate from the center
like pins stuck into a spherical pin cushion. Concretions vary in size
from buck-shot (buck-shot concretions in the soil) to oddities ten or
twenty feet in diameter, or even longer in elongate forms. The variety
about one-sixteenth or one-thirty-second of an inch in diameter is
called an oolite (pronounced oh-oh-lite). Some chert of southern
Missouri, most of the diaspore and burley clay, and a limestone cropping
out near Louisiana, Missouri, are made up partly to almost entirely of
oolites. See page 29.
Concretions may be composed of pyrite, calcite, limonite, chert,
cemented sandstone, or even cemented clay. They are usually recognized
by their structure after the previously enclosing rock has been eroded.
Thus pyrite, limonite, or calcite (limestone) concretions remain after
shale has softened and washed away, chert remains after limestone, and
strongly cemented “irony” (limonite or hematite) or siliceous sandstone
concretions may be found on the outcrop where the softer or less
resistant host rock has been carried off. The irregular-shaped,
intergrown, nodular limestone concretions (sometimes called
“loess-kinder”, or loess dolls) in the upper part of loess deposits
along the Missouri and Mississippi Rivers can be found remaining on
rain-washed slopes. Limy, mudstone concretions and brown iron carbonate
concretions are abundant in certain localities in northwestern and
southeastern Missouri, where they are used as oddities in rock gardens
or walls.
[Illustration: Concretions. Dark, limy concretion at left and brassy
pyrite at center and right. Note the inter-grown pair in center.]
Some concretions are formed at the same general time as the surrounding
rock accumulates, but others may be formed years after the surrounding
rock has been buried or removed from the environment of its formation.
In either case, deposition of the mineral matter follows the pattern of
addition or “growth” from inside out. This growth, of course, does not
involve a life process like that of a plant or animal. If two or more
centers of deposition occur close together, the several growing
concretions may touch, intergrow, and develop some weird forms,
suggesting organic growth. However odd these curiosities may become,
there is no question that they are not fossils, or evidence of life.
Probably concretions excite the interest of persons more than any other
rock structure.
Ground water, carrying calcium carbonate, silica, or iron compounds in
solution, is a great concretion builder. It percolates through sandstone
or other permeable rock and slowly leaves behind enveloping layers or
additions of mineral matter, until a concretion is formed, to remain
hidden from view until its host rock is softened and removed by the
action of the weather.