foot of a mountain
麓 centers on the base or foot of a mountain, where the slope meets the surrounding land.
麓 combines 林 (forest) and 鹿 (deer), likely depicting a forested area at the base of a mountain where deer gather. The modern form preserves this compound idea.
Picture a forest (林) at the bottom of a mountain where deer (鹿) roam: that forested base is the foot of the mountain.
For ロク, imagine a rocky foot of the mountain: rock -> ロク, and the deer stand on the rocky base.