Stonehenge and its Environs 134
Image by Chance

With the arrangement of two logs above the stone and two beneath as in our 2005 stonerowing experiment (see www.stonehengetheanswer.com ) click on 2005 experiment, levers can lift the upper logs and the stone can be lowered into the boat without the levers fouling the gunwhales.

It can also be done in tidal waters by "hanging" the stone over a dock and waiting for the tide to raise the boat up to the stone. With a skin boat it might even have been possible to "build" the boat around the stone.