It’s the sort of feature that gets astrogeologists exciting when they spot it on photographs of the surface of Mars or on the moons of the major planets as it’s evidence of flowing water (or on Titan it could be flowing liquid methane!).
It’s just a guess but I think that the block is now upside down and that originally this was the edge of a channel through a sandbank. If I’m right, the curve cutting through the rock represents the side of the channel, scoured out by a distributary stream in a river delta and the ripple marks show where, nearer the surface of the flowing water, sediment was redistributed to form the ripples.
These sandstone blocks were in the garden when we moved in and I suspect they came from the old quarry in Coxley Valley which is only a few hundred yards away. In the face of the quarry there are several examples of channels cutting through what were once sand banks.