Concretions
Concretions form within parts of already deposited sediment layers in which mineral cement, carried by groundwater, fills the spaces between the sediment grains before the rest of the sediment is hardened into a sedimentary rock. They are made of the same material as their host sediment but are cemented by harder minerals. Being more firmly cemented than the enclosing sediment, concretions weather out in various shapes.
Rounded stratified sandstone concretion
Size: 8 cm
Sandspikes
Unique type of sandstone concretions cemented by calcite
They were formed in a common sedimentary bed with the tapering spike-like ends all pointing to the same direction
Size: 15 cm
Septarian Concretion
It is an unusual type of concretion which is formed when cracks appear in the concretion due to shrinkage. The cracks are later filled with crystals deposited from percolating water, here they are filled with calcite.
Size: 25 cm
Limonite Concretion
Limonite occurs as a cement forming diagenetic mineral that precipitates from Fe-rich fluids.
Size: 20cm
Pyrite “Dollar”
Coal measures near Sparta, Illinois, USA
Size: 8cm
Resembling a fossil sand dollar (echinoid), this is a pyrite concretion in its natural radiating crystal form formed in the thin beds of black shale where the pyrite crystal grows outward in only two dimensions from the centre nucleus.