Among the most interesting types of stones are breccia and conglomerates, stones wherein a mix of small fragments have been compressed together in a fine matrix. In breccia, the fragments have not travelled far and so are angular and sharp. In conglomerates, the fragments have undergone some rounding from the travel they have experienced, usually from water, prior to being compressed. Sometimes the fragments are from different sources and are different colours and sizes; sometimes they may be from the same source. Stone of the Day #2 is most likely a breccia, consisting of fragments from the same source. It was found on Gemstone Beach sometime in 2021.
Another possible source of the fragmentation of this stone is crushing while in situ from physical forces. This means that the fragments would not have travelled at all.
This stone has gone through a similar tumbling process as Stone #1 – in 400 grit for about 10 days, then 600 grit for another 10 days or so, 14 days in tin oxide polish, and finally burnished in borax for 13 days. The 600 grit stage is a kind of pre-polish that I am adding to some stones, especially the kind of jaspers that can come out flat-looking and stones of complex composition where different parts may polish at different rates. Stone #2 has turned out very smooth and well polished.
I have come across breccia-type stones on Gemstone Beach before (see photos below). If all the fragments are rounded, the stone is a conglomerate but I have found very few conglomerates on Gemstone Beach. There are many stones there that seem to have a mix of angular and rounded fragments, making them difficult to categorise as one or the other. What might be the origin of these Gemstone Beach stones?
Many of the stones on Gemstone Beach originate from inland to the north, having been brought down to the coast by the Waiau River. The Waiau River mouth is about 12 kilometress northwest of Gemstone Beach. The river begins as the outflow from Lake Te Anau which itself is fed by a number of other rivers and streams, the two largest being the Eglinton and Clinton Rivers. The Upper Waiau River flows from Lake Te Anau into Lake Manapouri, 10 kilometres to the south, and from there the Lower Waiau flows further southwards for 70 kilometres before reaching Te Waewae Bay on Foveaux Strait. Water from Lake Monowai also flows into the Lower Waiau River, via the short Monowai River, about 20 kilometres south of Lake Manapouri. Lakes Te Anau, Manapouri and Monowai all originated from glacial action, being left behind as glaciers melted and receded.
Landslides in Fiordland, from many thousands of year ago, could be one possible source of the tiny rock fragments that end up in breccia and conglomerates. At the northwest end of Lake Monowai is the largest known landslide above sea level of its type in the world. Covering an area of 45 square kilometres and with an estimated volume of 27 cubic kilometres, the Green Lake landslide (map below on left) occurred about 12,000 to 13,000 years ago. It is thought that a nine kilometre stretch of the Hunter Mountains, heavily faulted from ancient earth movements, was undermined by a shrinking glacier and shaken by an earthquake (see “Nature and conservation” section here).
An excellent account of the Green Lake landslide can be found in Dave Petley’s Landslide Blog (Professor Petley is a geographer and earth scientist at the University of Sheffield in the UK). He notes there that the landslide is so enormous that early mappers failed to recognise it as a slide, with the first proper description of it as a mass movement occurring only in 1983.
The Green Lake landslide is one of 39 known very large postglacial landslides in Fiordland discussed in a 2009 academic paper by Graham Hancox and Nick Perrin, published in “Quaternary Science Reviews”. A 2013 Report published by GNS Science looks in detail at three giant landslides in northern Fiordland, including one whose debris covers over four square kilometres, crossing the Eglinton River and being responsible for the formation of Lake Gunn (see map above on right, also pages 20-23 of the Report). This landslide would have provided material to be carried down the Eglinton River into Lake Te Anau.
Stone of the Day #3 is here. The Index to the January 2022 Stone of the Day Series is here.
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