Standing stones

The mass extinction at the end of the Permian period, just over 250 million years ago is the greatest catastrophe that has ever hit life on earth. A perfect storm of volcanic activity, the breakdown of oil and coal deposits, huge releases of both methane and carbon dioxide from a number of sources, acidification of the oceans and an impact from space that created the biggest crater in S America, led to the complete loss of over 80% of the genera that then existed. Atmospheric carbon dioxide rose from 400ppm (a bit less than it is now, after our efforts over the last 200 years) to 2500ppm. It's astonishing life survived at all

The National Stone Centre is at Wirksworth in Derbyshire, on the site of an old limestone quarry. The local stone here was formed 300 million years ago, before the great extinction, and contains fossils that are much less familiar than those in the the post-Permian limestone of the 'Jurassic coast', or the Cretaceous limestone around our Oxfordshire home. A particularly striking one was something called a Crinoid - easily visible in the rock. In life, it was a bit like a starfish on a stem, attached to the ocean floor, and grew in large colonies. The explanatory text said they were grazed by sharks, which didn't seem to me like a fair fight

The Centre had a clever idea for the millennium. They built a winding, stone wall (of course) but invited each of the dry-stone-wall associations from all over Britain to build a section in the stone and style of their area. I can only imagine the disputes and feuds that must have taken place over exactly how a particular area should rightly be represented but, anyway, it got done, and makes an intriguing display. Much of the stone is dressed into cuboid shapes, before being laid, but the one in the foreground of this picture stood out for its large, water-weathered boulders, some almost spherical. The builder must have confidence in their ability to trust that the whole think is not going to roll off down the hill. This is the work of craftspeople from Dumfries and Galloway