Render horizon silhouette using Metal in SceneKit

Question

I'm working on simple side project and small part of it is rendering a terrain. I'm rendering the terrain using height map information. But here is my problem:

I would like to render just a silhouette/outline of the terrain/horizon. Here is screenshot from my app rendering height map:

And here is screenshot similar to desired result from peakfinder.org:

I would like to draw just lines representing silhouette of terrain with the rest transparent or solid colour. How can I solve it? Calculate local maximum somehow?

I created sample project here, in case you want to help me.

Thanks!