Drawing a curve, all points on z-axis. Want to disturb points using Mountain SOP. Nothing happens!

Moving a single point on curve off of z-axis by tiny fraction (0.00000000001) and it performs as expected.

Thought it may have been related to grid snapping but second curve created manually shows same behavior.

Hmmm… a little more testing: adding normals to original curve via Attribute Wrangle and Mountain works. So it looks like Mountain depends on either implicit or explicit normals to function.

Is this expected behavior?

Rebus B
Is this expected behavior?

Yes. From the documentation on the Mountain SOP [www.sidefx.com]:

This operator uses normal attributes on the input geometry. If a normal has 0 length, this operator will not displace the point along that normal.