How to mask or trim off the end of a widened path the the outline of another shape

[trailmapper]

I'm a beginner with Illustrator.  I have a widened path representing a trail. At the narrow (far) end of the path it extends beyond the outline of the mountain on which it is overlain. See image.  I want to trim that widened path to the outline of the mountain.  I can't figure out how to do this.  Have tried using shapebuilder but it won't let me select a path, even a widened one. I have tried trimming the path, but then it leaves a gap on the inside of the mountain outline.  Any suggestions?  Thanks.

[Jacob Bugge]

You are welcome, trailmapper, looking forward to the outcome if you will share it.