concretions are spherical to
oval-shaped calcium carbonate concretions that are flattened
parallel to the bedding planes. They can range in size from
3 cm to 8 m in diameter. They most often precipitate around
a nucleus of fossilized material including plant matter, shells,
or even remains of fish. Often when the concretions erode,
they can form odd shapes, sometimes resembling fossils. These
are very common in the Devonian rocks found in the Ithaca
concretion found in the Hamilton Group of Ludlowville, New
York from the Middle Devonian. The nucleus in this case was
the fossilized remains of an ammonoid. PRI ACC 1107)
concretion from the Middle Devonian. PRI ACC 1053)
calcareous concretion from central New York. The cracks in
this concretion occurred as a result of the collapsing of
the dome which often occurs in the larger concretions.)
nodules are distinctly sphere-like concretions that
are characterized by a series of cracks that widen towards
the center and die out towards the sides of the concretion.
These radiating cracks are often crossed by a series of concentric
cracks giving them a "turtle-back" appearance. Dehydration
of the concretion creates the cracks which then are filled
with another crystalline cement, such as calcite or silica.
They can range in size from 10 to 100 cm in diameter and usually
are made up of a large component of iron.
nodule found in the Windom Shale of Ludlowville, New York
from the Middle Devonian. PRI ACC 1056)
Septarian nodule found in Crowbar Point near Cayuga Lake in
concretions occur along the bedding plane as a 2 to
15 cm thick layer. It is composed of an aggregate of upright
cones with ribbed or grooved sides, usually made up of minerals
like calcite or gypsum.
of cone-in-cone concretion from Sedimentary Rocks by