I thought that last night, wet chalk is amazingly slippery, their announcement that they were "natural" put the thought to bed for me, i must stop listening to "experts."

the problem is the sarsen used to make up bridge the isn't even enough for a large single stone.
The bridge support on the land side was only 2-3ft high and they ran out of sarsen to make a complete central support as it is packed out with flint nodules.
There was no stone on the riverside.
If there were four stones you would expect to see a lot more in the area.
I searched the riverbank in winter and couldn't find anymore.
Pete