@Blattsalat:

I guess you are trying a tredmill approach (otherwise I would advice you as well to use a large (chunked) terrain.

If you would stitch both terrains in MED, the gouraud shading would do the right shading. Now, lets take that hill on your image and slice it: there are two models, forming a hill from the left with this slope _/ and from the right with this slope: \_ Obviously, the shading in the concave section of this slope is correct, but you the real slope of 45 degrees or so the one side is shadowed and the other side bright. There is no smooth transition.. but this is right: the gouraud shading doesnt know the continues way the slope would go from one model to another. So there is only a sharp edge. And so, there are no information to argue a smooth transition. The normals of the _vertices_ arent right because you cut the important ones.

How to cirumvent this:

Option A: use a large (maybe chunked) terrain
Option B: instead of slicing the terrain 100% accurately, add some "frame" around each tile, so that follow-up vertices are know so that you deliver these information. The additional vertices are hided by transparent skin parts or a tricky UV mapping
Option C: do it the way JCL mentioned: disable ANY gouraud shading and add a _precalculated_ shadow map instead. Then, you have no shading issues anymore.

I hope, you have got the clue..

Cheers
Christian