The sculpture is created by Duncan Elliott http://www.duncanelliott.co.uk/index.php?menu_ID=2 with Lias rocks found on the Mendip Hills. No chisel or any other cutting tool is used, just his eye & some cement to hold it all together. He has an exhibition on in the grounds of the Bishop's Palace, Wells, at the moment.
Lias is a form of Limestone, and is found in blue, grey & white.
Here's an interview he gave http://www.stone-ideas.com/2012/10/01/art-dry-stone-walling-in-extreme/