Friday, 25 February 2011
A Story in Stone
While fitting together the angular stones to make our new patio, a task that resembled the assembly of a large jigsaw puzzle, I began to dream about the stones' fabulous story. “If those stones could talk!” --- Who said that? Who said that they can’t?
We found them piled up on the side of a drainage ditch. The County, which occasionally spends a penny of our Council Tax to benefit us directly, sent in a backhoe to dredge the ditches that line our main road. They were clogged with mud and stone. Frequently overflowing after it rained. The backhoe excavated mounds of slate : dark blue rocks that cleave naturally along mineralized layers; also some pink, streaky quartzite. Between 3-5 inches thick, the rocks are perfect for laying out a pathway or a patio. For centuries the farmers used them to build walls separating various pastures. Charles A. came by with a trailer. We hauled several piles off to Cottarton.
The rock hadn’t grown in the ditch or anywhere nearby. There isn't a slate outcrop for miles. The closest outcrop is of Archean quartzite, a couple of miles up the road. It may have been the source for some of the rocks. Archean rocks formed around a billion years ago; long before the first shell fish and creepy crawlies evolved in our oceans. Life had already begun in the form of bacteria and the first cellular plants and animals, those that don’t leave any fossils.
Our slate may have crystallized around that time. But imagine in what forge the stone was melted and cooled. The dark minerals, amphibole and hypersthene that give the stone its colour form only under intense pressures and high temperature, not found anywhere close to the surface of the Earth. You have to dig 15 miles down before you find such conditions. Close to the Earth’s upper mantle. An unnamed rock, could be sandstone,shale or granite, was partially melted there. Under intense pressure little mica type minerals crystallized to give the rock its streaky fabric. Its cleavage. Then the rock was brought up to the surface. Not by a volcano the way basalt or granite are exuded but by active faults, fractures in the Earth's crust, moving over millions of years.
How did the slate fragments reach Cottarton? Either on the back of a glacier or pushed by a one as it advanced from the north, scraping up any rocks in its way; like a snow plough with a blade several miles long. That was recent history, within the past hundred thousand years. Due to small changes in the Earth’s orbit, the planet cooled a few degrees --- not so much as you might notice over a few winters, but enough to trigger an ice age. Starting in the Arctic, glaciers formed and migrated southward, carving the valleys that we admire so much these days. The Scottish Highlands, Kings Canyon and Yosemite. The slate and quartzite outcrops were crushed, their pieces churned up and pushed along to fill the valley floors. Smaller pieces altered chemically to form clay minerals: the heavy soil where today I grow vegetables and flowers. Fodder for our buttercups. I swear that the resilient buggers lasted the entire ice age under the ice and were the first to pop up once the ice disappeared about 12,000 years ago. If the ice couldn't kill them, what hope have I?
And so the forests returned to an altered landscape, the rounded hills and valleys of today. Great Britain, which had been connected to the rest of Europe became and island. Stones whose story began over a billion years ago lay fragmented in our glen until the backhoe dug them up. Now they grace our patio. Treat them with the respect that the grand old rocks deserve.