Valleys are very easy to make. I will just briefly explain how to make one quickly. This technique works for rivers, mountain sides and of course valleys :D
1. Click the Terrain Tab which will open the Terrain Editing Tools panel. Click the Sculpt tab. Click Mountain. Brush Parameters are Size, Strength, and Falloff which I have coded for this tutorial as S- size, O- strength and F- falloff.
3. Turn on Fog, if it's turned off (I usually work with it turned off for clarity)
7. When have gone all around, you should have something like this below.
Here is a view with the Fog turned on. Valley complete.
8.Although this tutorial is not for Terrain painting, I shall explain a quick technique using the default terrain paints. Click the Terrain Tab which will open the Terrain Editing Tools panel. Click the Paint tab. Double click sand_dark_base.dds texture and set brush to s-40 o-5 f-50
Click and hold mouse and do one even coat all round the valley
Tip: to keep it even all through. Once you click, hold the mouse until you are done painting. If you let go just make sure you start from an un-touched spot to avoid too much over-lapping of paint
And that's all there is to. I try to do things with as few steps as possible. But with this technique you can make more elaborate creations.
2. Your valley may be smooth or rough. Just adjust your Strength accordingly.
3. Depending on the Paint/Texture you are using, you may have to adjust your Strength as some paints are darker than others.
4. Always Brush up and down, not side to side as this follows the natural shape of the valley.
5. You may also use the mountain tool to create the ridges, depending on the effect you are going for.