OK  this is a guess, because I have tried it myself.
The beach itself is a custom polygon made to fit. The terrain on top of the cliffs is created with multiple 16N flat areas to control the elevations. [A terrain mesh editor would work better--I have not found one yet]. The white cliffs are polygons turned ninety rotated ninety degrees to make them vertical. They used a rock texture to cover the faces of the cliffs. I use FSSC to make stuff and that is the way I would have done that. There are probably better ways.