Fantastic advice! I only end up with the 'collapsing to a point' challenging geometry because I generated the sharp carved out part with an extrusion, bool, and then subsequent addition of a curve (valley center) and surface generation from there. That leads to 'sharp' edges. It did not occur to me that I can use another surface to trim the existing surface, thus creating already the valley profile I actually want..... From there, controlling the 'rim' of the valley is much simpler and straight forward....
Thanks a lot for your comments!
Lemo

Nurbs are tricky things isn't it? :)

You're welcome Lemo!

There can be many situations where it can be easier (and produce more reliable geometry), to kind of imagine a larger extended surface that gets trimmed back to make your final shape, rather than trying to create a surface only by following outline curves.

If your outline curves have a very natural 4-sided type layout, then surfacing them directly is fine, but otherwise building extended and trimmed back can be better.

Here's another example from a recent thread: http://moi3d.com/forum/index.php?webtag=MOI&msg=1127.4

In that case it helped to build a single larger slightly curved surface as the cap of the object which then gets trimmed, rather than trying to sweep using the existing outline curves which came to a kind of difficult corner which caused a sort of bunching and overlapping of the surface when trying to sweep.

- Michael