This image is particularly hard to evaluate. It has been rotated by 90 degrees clockwise, and shows a stone protruding from the wall immediately to the left of the West Kennet Long Barrow's entrance. I originally interpreted the stone as representing a snakes head, but rotated I see a face set in an elongated skull. I am reminded of Dr Thurnam's 1863 dictum "long barrows, long skulls; round barrows, round or short skulls".

I post this image with some trepidation, I hope that members of the UFO community don't cite this image as evidence that Avebury was built by aliens! ;-)