In other words, it never stood up on end.
Quite.
Possibly.
What Inigo saw, or thought he did, was the remains of a Roman altar - something he would have been very well informed about. They weren't all upright. http://216.239.59.104/search?q=cache:ZTW_rsBX3TIJ:community.iexplore.com/photogallery/displayFeaturePhoto.asp%3FID%3D144198+roman+altar&hl=en&ct=clnk&cd=21&ie=UTF-8
and some even had a groove - http://216.239.59.104/search?q=cache:YPncHR0gWTQJ:lexicorient.com/egypt/luxor06.htm+roman+altar&hl=en&ct=clnk&cd=8&ie=UTF-8
Anyone fancy checking out his sketches and notes from Stonehenge and his sketches of his stage designs? http://ipac.nal.vam.ac.uk/ipac20/ipac.jsp?session=1172N0187K833.1427&menu=search&aspect=basic_search&npp=10&ipp=20&profile=nal&ri=&index=NA&term=jones%2C+inigo&aspect=basic_search&x=16&y=9#focus#focus#focus#focus