Mountains
noun, の adjective
The mountain peaks are just the mountains. Just because you can only see the peaks doesn't mean the rest of the mountain doesn't exist.
さんがく
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
この山岳地帯では、少なくともあと三ヶ月は冬の季節が続くだろうね。
Winter will continue for at least three more months in this mountain district.