This chunk of sandstone, one in a ring around the ashes (see 24th January) of a bonfire has been reddened at the side nearest the heat. You can see from my sketch that although the lower part of the rock is weathered, the top facets are freshly exposed; heat has shattered it.
Two small fragments lie just to the right of the ring of stones while another two larger shards lie 11 feet away on the grass. Were they flung there when the stone exploded?
I guess that it was moisture turning into steam in the pores between the sand grains the rock, or in planes of weakness running across the rock, that shattered it. There's a slight speckling on the freshly exposed facets that makes me wonder if micro-organisms such as single-celled algae or bacteria might have already colonised a microscopic fracture line within the rock.
Like the block they were chipped off from these fragments are reddened, where the heat has oxidised iron salts in the matrix of the rock.
In the debris by the campfire there's another sign of the intense heat; partially re-melted bottle-green glass. It sparkles in the sunlight which picks out bubbly, wavy facets in the shards, giving an impression of translucent bubbling sea water. The misshapen fragments have an ashy frosting on their streaked upper surfaces resembling birch bark.