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?
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.