How to make slopes / gradation of vertical cliffs?

I have some heightmaps with almost vertical cliffs that I want to smoothen into even slopes. I don’t need any detail, just smooth slopes from the edge. I have tried with thermal erosion, but either the slope starts too low or there is an odd edge generated above the edge of the cliff.
I have included an image from Unreal Engine 4 since the effect isn’t visible in WM.
Can anyone give me some pointers? I use a height selector to select everything below the edge of the “cliff” and I don’t understand why it creates an edge above the selected area. :cry:

May I have a look at that heightmap?