Glendonites are the pseudomorphs of crystals and crystal clusters which grew in organic rich mud on the sea floor when the temperature was below 5 degrees centigrade. As the temperature rose and the original crystals dissolved, the space that they had occupied was colonised by harder minerals such as iron carbonate. Their presence in the Shoalhaven sediments is evidence of cold seawater temperatures when the sediments were laid down around 270 million years ago.
Glendonites were first described at Glendon, east of Singleton, more than 150 years ago. They were not understood until crystals of hexahydrated calcium carbonate were discovered growing on the floor of the Ika Fiord in Greenland in the 1960s.