In the terrain editor, you have an icon called "filter altitude" you can use for a regular terrain. If you use a procedural terrain, this filter is located in the "proc" ta, right under the shpere. To edit this filter, right click it or ctrl/click it. Then click on the curve and click "new keypoint". This will create a point along the spline, create another one and make the line joining the 2 points horizontal. Click preview and see what is does to your terrain shape. refine the filter if need be. Check the image I provided.