: http://www.qmuseum.qld.gov.au/inquiry/leaflets/leaflet0006.pdf
(thank you, Dean)
Cone-in-cone structures are very common in rocks throughout central Queensland and are commonly misidentified as teeth, fossil footprints or shells. The structure takes its name from its appearance as upright circular cones superimposed into one another so that their edges are ridged and grooved. The cones are usually at right angles to the sedimentary layering or bedding. The ridges on the side of the cone become less pronounced near the apex. The material that forms the cement mineral is usually calcite or gypsum. Growth of fibrous crystals in layers results in vertical pressures in the rock causing small breakages. As pressure continues, the breakages overlap causing the stepped-cone structures.The cement minerals dissolve leaving ridges along the cone surfaces.
Septarian nodules are concretions that have a series of radiating cracks, crossed by a series of concentric cracks to form a ‘turtle back’ appearance. Indeed they are commonly mistaken for fossil turtle shells. These form in a similar way to other concretions, that is by cementation, but in their case the cement minerals (clays) contain water. Other processes dehydrate the concretion to produce the cracks that are subsequently filled with another crystalline cement mineral. These nodules are more resistant to weathering than the surrounding sedimentary rock and remain behind after erosion of the surrounding rock.
