Well, at first you have a flat terrain.
You add a fractal terrain - go for old_blaggards idea - if this is'nt fine enough, try that coastline thingy, I told you, too.
Else, in the fractal terrain, at the very bottom you can turn on blend by shader. Create a new Powerfractal and use this as blend shader. Try settings:
Feature scale 10,000
Lead in 5,000
smallest 4,000
in the second tab reduce the colour roughness. Then reduce it again ,-)