The Fingernail Test

Monday, 10th April 2006

CastletonAt last, a decent day, so I've come to Cavedale, Castleton. While I'm drawing the medieval castle, I look down and see a sharply defined pattern of concentric rings in a fragment of limestone at my feet.

Later, I e-mail my field sketches to a geologist friend who tells me that 'the flat shell looks like a spirifer but I am not sure about the +ve and -ve shells.'

' The mineral is interesting,' she says, 'If you could scratch it with a fingernail then it was probably gypsum not calcite which is odd. But I have found some strange minerals in Castleton - it is famous for them.'

brachiopods Life on the Reef

I must go back and take a closer look at the minerals. I always find the fingernail test hard to judge. Is the fingernail really scratching the mineral or is the mineral scraping the fingernail, leaving a white mark on the mineral? Calcite is the most likely, so perhaps I was mistaken. Calcite would scratch with a knife but not with a fingernail.

I think both species of fossils are brachiopods (of which Spirifer was a species), a creature with a thick shell with hinged valves, not related to the sea shells that we're most familiar with, which are molluscs.

If I'd been here 320 million years ago I would have been, as I understand it, in a surge channel at the edge of a reef. The castle stands on the top of the reef. To the south there was a tropical lagoon (Britain was on the equator at the time), while to the north, in a similar topography to that seen today, there was deeper water.

deltaFrom the north a river delta was advancing, as grit, sand and mud poured into the sea, washed down from a mountain chain that stood where the Scottish hills stand today. Next Page

Richard Bell,