This is my sixth contribution to the current Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “F is for Fossil Worm Casts” (referring to trace fossils).
F is for Fossil Rhodolith in Fossiliferous Limestone
I found the stone above, as well as the one below, on a Kakanui (North Otago) beach in September/October 2024. They were identified for me by a fellow member of the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils” who I met on the beach. The rhodoliths are the larger spherical features. Fossiliferous limestone is a type of limestone that contains noticeable quantities of tiny fossils or pieces of fossils – see Geology.com on the different types of limestone.
Rhodoliths are marine nodules of algae, made up mainly of calcium carbonate, that can grow to resemble beds of coral. They roll like tumbleweeds along the seafloor until they become too large in size to be moved by the waves and current. A field trip guide on Coastal Otago, written by Daphne Lee for the 2009 Geosciences Conference, describes the fossil rhodolith beds in the Kakanui area and the Waitaki Museum in Oamaru contains specimens of fossil rhodoliths – see page 12 here. For more, see “September 2024 Stone Collecting Trip –Part 24, Fossil Rhodoliths and Farewell to Kakanui’s Seadown Beach, Tuesday 1 October”.
The next entry is “G is for Granite”. An Index for this Alphabetical Series is here.
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