I would slice out a section and insert the staircase- but it is not so
simple as that for most isosurfaces.
    My solution is to render the isosurface in parts.  I create a container that
bounds the isosurface so that I don't get spurious objects or "dust" from the
function.  Then I insert my object.  For your mountains, it may involve two or
three segments.
    So imagine the staircase as a rectangular block- it is bounded by perhaps
three sides that will be in the scene.  Make a left, rear, and right isosurface
"chunk" so that each reaches its container right at the edge of your stair, but
overall they will merge into a single mountain range when rendered.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

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