The process is called head cutting. Will continue until it establishes a stable slope. Your conservation district or NRCS office should be able to give you some guidance. Placing R-4 Rip Rap is a standard practice but the size stone needed is dictated mostly by the size of the watershed and the shape of the channel you create with the stone. Creating a shape with the stone is the most important. Just dumping the stone will only cause the water to run around it. You still need to create a low point (swale) with the stone so that the water runs over it, not around it....