Wednesday, August 11, 2010 Fossil of the Week 8/11/10 -
Concretion Although this might look like a fossilized turtle
shell, it is neither a turtle nor a shell. It's a concretion,
and a big one, approximately 30 centimeters (12 inches) in
diameter. Concretions are not fossils at all. They are common
geologic phenomena found in all types of sedimentary rock,
especially the shales and siltstones of central New York, so
they are quite common in the Ithaca area. Concretions form when
minerals (such as calcium carbonate, which forms limestone)
within soft sediment precipitate in concentric layers around a
nucleus, such as a shell or pebble. So a concretion often
actually contains a fossil. Concretions are spherical to oval
masses, and are harder than the surrounding rock, so the rock
weathers first, exposing the concretion. They can range in size
from less than an inch to several feet in diameter. Sometimes
the concretion will crack and the cracks will fill with matrix,
forming a rounded rock that looks like a turt