There is always the possibility that the smaller ones are natural and the larger ones are enhanced. Many bullauns are by water, so I have no problems with seeing these ones next to water.

I have no doubt that some bullauns were actually formed in river beds by tumbling rocks and moved to the shore for use. There are still places where 'bullauns' in river beds are thought to be some saint's knee-marks made when he baptised the local king.