Concretions are hard bodies that form in sediments before they
become sedimentary rocks. Slow chemical changes, perhaps related
to microbial activity, cause minerals to come out of the
groundwater and cement the sediment together. Most often the
cementing mineral is calcite, but the brown, iron-bearing
carbonate mineral siderite is also common. Some concretions have
a central particle, such as a fossil, that triggered the
cementation. Others have a void, perhaps where a central object
dissolved away, and others have nothing special inside, perhaps
because the cementation was imposed from outside. A concretion
consists of the same material as the rock around it, plus the
cementing mineral, whereas a nodule (like flint nodules in
limestone) is composed of different material. Concretions can be
shaped like cylinders, sheets, nearly perfect spheres and
everything in between. In size they can range from as small as
gravel to as large as a truck. This gallery goes from small to
large.