Plural nouns are regularly used throughout the poem:
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
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.