"Frequently within 15 feet to 100 feet of the terrain, "
In a helicopter I hope.
What were you doing this close to the ground?
Mountain flying, probably leisure, or airborne geophysics, or maybe firefighting, or SAR, or military.
Note that the terrain clearances mentioned could equally be vertical or lateral. E.g., One (and perhaps the only) way to cross a col might be to catch rising air flowing next to the slope (orographic or anabatic lift). Also, if you think you might need to do a 180, you get as close to one side of the valley as possible (the downwind side usually) in order to leave as much room as you can towards the turning side.
btw, I concur with the spirit of SNS3G's last paragraph in his previous post, as mountain flying is an entirely different discipline compared to "normal" flying. It's a bit like questioning what was a seaplane pilot doing so close to the water, or what was an aerobatics pilot doing flying inverted.