Hi there.
I was asked how a clip file worked to flatten out valleys recently. Unfortunately my previous math didn't work too well. The idea then was to square root the displacement below a certain level. Ie, make valleys flatter. I'm sure it could work with a little more applied grey matter on my part (it's Friday night), but in the mean time, I've uploaded another approach.
Here's a clip file, which has three variables. "Start of leveling" is the altitude you want the landscape to start curving towards your "Base Level". The "Blend Curve 0 to 1" defines how the blend curves towards your base level. I'd suggest numbers between 0.1 to 0.5, but that's entirely up to the effect you want to achieve. Have a look at the Bias Scaler node in the reference to see how values work here.
http://www.planetside.co.uk/docs/tg2/noderef/window_1_13_1.html?MenuState=fABEAAQABAAAAQEABAAAAABAQAEAEAcVVVVABEEAEAQdVVVVXdVVVBAEAAAAEAAAAAAAAAAAAAAThe only thing I would warn, is that this uses the Get Altitude node, which takes it's measurement at render time, so any displacements you make after this clip file will be taken into consideration. To over come that, you can use a PF attached to an Add Scaler to modulate the Get Altitude before it's connected to the Linear Step node. This modulation will use colour values instead of displacement, so tap in colour values of meters instead of 0-1, and dial back the colur roughness of this PF towards 1. This will modulate the altitude, so with tastefull values will give you subtle undulations on your valley floor.
All the best
Jon