I'm not sure about "mountain men" - the time period I'm interested in is a little earlier (1760's & 1770's) - but the white woodsmen did adopt some of the native Indian dress. You can't see it but I'm wearing black wool leggings over linen knee breeches in the photo. Leggings would have been worn with breech clouts (loin cloths) as well and held up by tying the tops to the belt that held the clout. A garter was also tied below the knee to keep the leggings in place. (My garters are smoked braintan with white quillwork.)
If you want to be "period correct", try to narrow down as small a date range as possible as well as a specific geographic location.
There are generally two types of leggings... side seam and center seam (Side seam were more common in the east.) - and wool was very commonly used for them. Leggings are real easy to make... they go from mid-thigh to the ankle...
Leggings