Greywacke – NZ’s foundation

Greywacke is the base rock of New Zealand’s South Island. Along the west coast of South Island the Australian Plate is being forced under the Pacific Plate resulting in the mountains known as the Southern Alps, as materials are accumulated and forced upward. This is a mountain range that is growing at about 3 inches a year but not really gaining in height as it is eroding at about the same rate. Greywacke is the stone that is being bulldozed to form these coastal mountains. It is a young stone and was a bit of a puzzle for quite a while. The stone is made of rough sand, muds, and clays; at least 15% clay. These sediments should have settled out of a water (flow) stream at different places with the heaviest dropping first and the clay stuff dropping last, usually in very still water. But they are all mixed in greywacke – how?

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Greywacke falls apart easily. Water, heat, and especially glaciers really break and grind this stone down into bits and pieces. Here walls of greywacke are shedding pieces that have fallen into the drainage below. Water will then carry the debris lower and lower in the system until the fines are dropped in a lake or perhaps a fjord.

It is probably the result off old underwater mudslides that brought all the stuff down at the same time in a great storm of mixed materials. Then time, compression, and intrusion by quartz and after a while you get greywacke. Our mountains in Glacier National Park are not exactly the same material but they are also of a loose shale sort of rock and are crumbling and washing downhill as rapidly as the Southern Alps. It seems that nature and time merely want to wear down the mountains, fill in the valleys, and make the planet smooth.

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The quartz in greywacke can be very obvious. It has intruded into the thin cracks or between the sedimentary layers of the mudstone.
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This is a wall of greywack about ten feet tall. It has been shaped by a glacier that was flowing from left to right and scouring the greywacke as it passed. In this instance the original stone was laid down in horizontal layers which have been turned 180 degrees by tectonic action, so they are now vertical.

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