Inspired by a poll, everyone who follows me gets to look at a rock per day for the month of October!
Day 12: Today’s rock is a graywacke and a little longer of a story! A lot of different rocks, minerals, textures and features in geology are named by where they were first described – in this case it is from the German “grauwacke”. Graywackes are a type of sandstone that forms in landslides deep underwater, so they show layers and currents despite their jumbled nature.
The graywacke in these pictures is in Japan, in a mountain named “Nokogiriyama” or “Saw-Toothed Mountain” that is right across the bay from Tokyo. Edo (now known as Tokyo) needed a lot of stone for its walls and streets. The stone needed to be durable, but soft enough to easily carve with hand tools. The graywacke was perfect; an entire mountain of material that could be cut with large saws and loaded onto barges for transport across the bay. It was an industry, and over centuries the mountain was carved into manageable blocks and its distinctive profile. The cut marks are visible in every surface and piles of cut rock still remain.
Eventually, power tools entered Japan, and the quarry was closed in favor of harder rock. There is no longer any trace of the mined stone in Tokyo. It is under the rubble of building and rebuilding a city that has gone through major earthquakes, firebombing during World War II, and a massive modernization effort after the war.
All that remains is a quiet mountain covered in the carefully straight lines.
