The speaker of the poem may be thinking back to his childhood and how he used to view climbing his grandfather as climbing a mountain. Another interpretation could be that as an adult, climbing a mountain now reminds the speaker of his grandfather. Either way, Waterhouse presents the reader with an extended metaphor, which compares a kindly old man and a craggy mountain.
There is a sense that the speaker enjoys free-climbing and mountaineering as an adult. Through imagining climbing his grandfather as a mountain, the speaker is able to recall clear details from his childhood of scaling a great man. Although he may have lost his grandfather, these memories help the speaker to feel closer to the man he knew.