I use pea gravel for harling, which is almost pebbles.
In Scotland they use hydraulic lime for harling, so it would be pretty tough. whether it be with gravel, pebbles or rough-stuff. The last roughcast I did was with NHL3.5, so there's little chance of that being washed away.
And as for liking the sound of Roger - who doesn't? But this is a sensible, family-oriented forum. Or maybe it's time I got my coat...
Well I thought harling contained pebbles, but I tried to find info and nothing said so. I need a trip over the border obviously on a fact-finding mission.
The Scottish Lime Centre I have no doubt will have info...