Full Idea
It can be perfectly determinate which mountain x is without x's extent's being determinate. A mountain is not, after all, something essentially demarcated by its extent or boundary.
Gist of Idea
Boundaries are not crucial to mountains, so they are determinate without a determinate extent
Source
David Wiggins (Sameness and Substance Renewed [2001], 6.5)
Book Reference
Wiggins,David: 'Sameness and Substance Renewed' [CUP 2001], p.166
A Reaction
This endorses something I have always wanted to assert ('a vague boundary is still a boundary'), but with the interesting addition that one might think about vagueness in terms of what is essential to a thing. Hm....