The stone may simply be a glacial erratic with no archaeological significance and it is not listed by the RCAHMS or mentioned by the outstanding local archaeologist in Victorian times, John Smith.
This was said to celebrate the crossing of the Irvine at this point by William Wallace in 1297 on his way to attack the English army.
The episode in 1297 relates to a punitive expedition under Sir Henry Percy sent to Irvine to quash an armed uprising against the dethronement of John Balliol.
The Earl of Carrick, Robert Bruce and others led the Scottish army, however after much argument they decided to submit without a fight.
The story became embellished with a purely fanciful involvement of William Wallace in a brave action here.