Etymology
These large rocks referred to locally in Richmond as ‘moonstones’, ‘moon rocks’, ‘pudding rocks’, ‘snow men’ and ‘dinosaur eggs’ are a type of geological phenomenon called a concretion. The word 'concretion' is derived from the Latin ‘con’ meaning 'together' and ‘crescere’ meaning 'to grow'. These rocks get their local name from their round shape and white colour resembling a full moon.
These concretions should not be confused for the gemstones also called ‘moonstones’, which are sodium potassium aluminium silicates, typically made of the mineral Adularia.
Discoveries
Reports of concretion dating from the 18th century attest to the fact that these strange rocks have long been regarded as geological curiosities. Unfortunately there are no record detailing the exact date concretions in Richmond were discovered or named ‘moonstones’. However it certain that they would have been a prominent feature of the landscape for the early pastoralists in the region. The earliest scientific report of these local concretions is in the 1959 definition of the Toolebuc Formation (previously called the Toolebuc Limestone Member). This geological formation is the source of these concretions, which typically form in particular stratigraphic intervals (layers of rock) with limited horizontal extent. Subsequent to their original description, little other scientific studies have been conducted on these odd rocks.
Mr Gary and Mrs Barb Flewelling resting on some moon rocks in the Toolebuc Formation.
Description
Most moonstones are rounded or ovoid in shape, although some have multiple concretions fused forming strange dumb-bell or snowman-like structures. They can also vary in size – some can be as small as a golf ball, while others can be as large as a car. This is due to natural chemical processes which occur during formation and is not as a result of weathering as might be expected.
Detailed studies have demonstrated that concretions form after sediments are buried but before the sediment is fully lithified (turned to rock) during a process known as diagenesis. Commonly concretions ‘grow’ through chemical participation (normally of calcium carbonate) starting from a nucleus such as a grain of sand or a fossil fragment at the centre. The cemented zones continue to grow from the nuclei, and may grow rapidly or slowly depending on conditions. Some also show clear evidence of the cementation processes restarting after a pause and change in conditions. The precise detail of the mechanism of cement growth is complex and may involve the help of microorganisms like cyanobacteria. Since the sediment is typically uniform in all directions, the concretions form roughly spherical shapes.
Because of the variety of unusual shapes and sizes, concretions have previously been incorrectly interpreted to be dinosaur eggs, animal bones or even human artifacts, making them ‘psudeofossils’ (literally false fossils). Pseudofossils are inorganic objects, markings, or impressions that might be mistaken for fossils. These false fossils may be misleading, as some types of mineral deposits can mimic lifeforms by forming what appear to be highly detailed or organized structures.
Although concretions are not fossils themselves, they can sometimes contain fossils featuring exceptional preservation. Around Richmond this has commonly included bivalve or ammonite shells. More rarely, ‘moonstones’ can contain the three dimensional fossilised bones of fish and marine reptile, so with soft tissue also preserved.
An enormous moonrock excavated by Dr Troy Myers. This specimen contained the skull of an adult ichthyosaur and was so heavy that it had to be excavated by a loader. The skull contained inside this moon rock is now on display at Kronosaurus Korner.
References
Al-Agha, M.R., S.D. Burley, C.D. Curtis, and Esson J. 1995. Complex cementation textures and authigenic mineral assemblages in Recent concretions from the Lincolnshire Wash (east coast, UK) driven by Fe(0) Fe(II) oxidation. Journal of the Geological Society 152: 157-171.
Casey, J. N. 1959. New names in Queensland stratigraphy (Part 5) North-west Queensland. Australian Oil and Gas Journal 5: 31-36.
Cook, A. and McKenzie, D. E. 1997. The Great Artesian Basin. M’Choinneach publishers, Illfracombe, Queensland Australia. pp. 53
Davis, J.M. 1999. Oriented carbonate concretions in a paleoaquifer: Insights into geologic controls on fluid flow. Water Resources Research 35: 1705-1712.
McBride, E.F., M.D. Picard, and K.L. Milliken. 2003. Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A. Journal of Sedimentary Research 73: 462-483.
Mozley, P.S. 1996. The internal structure of carbonate concretions: A critical evaluation of the concentric model of concretion growth. Sedimentary Geology 103: 85-91.
Mozley, P.S., and Burns, S.J. 1993. Oxygen and carbon isotopic composition of marine carbonate concretions: an overview. Journal of Sedimentary Petrology 63: 73-83.
Mozley, P.S., and Davis, J.M. 2005. Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale microbially induced, chemical heterogeneity in groundwater. Geological Society of America Bulletin 117: 1400-1412.
Raiswell, R., and Q.J. Fisher. 2000. Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition. Journal of Geological Society of London 157: p. 239-251.