Word choice
Plural nouns are regularly used throughout the poem:
mountains
peaks
hills
This highlights the extent of the mountains encompassing the landscape, building an impression of them as being abundant (and perhaps infinite).
As well as portraying the mountains as being plentiful, they are also depicted as powerful. This is particularly apparent in the first stanza, where MacLean refers to:
a great garth of growing mountains [...] coming on with a fearsome roaring
Garth
is an enclosed piece of land. The power of the growing mountains
is made clear with the word garth
. Their imposing nature is consolidated with their fearsome roaring.
murmuring bareness of marching turrets
The landscape is presented as a united force. At times, the mountains are portrayed as organised, with their edges providing natural protection for people, like the turrets of a castle.