Quote Originally Posted by Caenwyr View Post
If the rocky area is relatively small, I'd think the river would flow around it, and not over/through it. There's no reason for a river to start eroding a rocky outcrop if it can avoid it: water generally follows the path of the least resistance ;-).
The idea is that the river flowed as normal through the path of least resistance, made of regular soil. However, by eroding the soil at a certain point it exposes the rocks underneath, which are not smooth but have "spikes", thus the rocky bottom and the small islands. This entire rocky formation relatively near to the surface intersects the flow of the river more or less perpendicularly, thus the river does not go around it but through it, after having eroded enough soil to uncover it at a certain point.