How would you feel if you owned an entire mountain? Now, just think if you came from a legacy of slavery and were never allowed to own land in the past. How would you feel then?
If you're anything like the Higgins, you just might feel extreme devotion and loyalty to the land, because not only does the mountain house your ancestors who seem to live on as ghosts (M.C.'s constantly hearing and feeling them around him), but the mountain also signifies your freedom.
For this reason, Sarah's Mountain isn't just a symbol of freedom; it's actual evidence of freedom.
That's why one of the important things Jones points out about Great Grandma Sarah is that she got the land deeded to her, a deed he now owns. He tells M.C.:
[…] there's an old title I have to this mountain slope. Show it to you sometime. Says deeded fee simple from McKelroy lands to Sarah McHigan, 1854. (4.67)
As for how valuable this is to the Higgins family, Banina notes, "It's good when you own […] Least the roof is yours, no one can take it" (6.98). This stands in stark contrast to a life lived enslaved. It isn't just the roof that can no longer be taken from the Higgins folks, though—it's each other, too. And because of this, the Mountain represents the Higgins family's ability to stick together and put down roots, to build a family history as well as a family home. And it's this history that M.C. taps into because, you know, he's basically one with the Mountain.
So think of the Mountain as another extension of M.C., a huge one, and think of M.C.'s character as an extension of the Mountain's history. (By the way, if you've looked at the other symbols in this section, you'll notice M.C. has a lot of "extensions" of himself. What can we say? M.C. is the main guy in the book.)
More than any of the other Higgins, M.C. seems to belong to the Mountain. He has a vision of himself in the future, an old man who "[n]ever did leave the mountain" (2.8). That vision turns into "a trance":
M.C. gazed out over the rolling hills. He sensed Sarah moving through undergrowth up the mountainside. As if past were present. As if he were a ghost, waiting, and she, the living. (2.15)
Which is why M.C., like Jones, wants to stay on the Mountain in the end. Why he's willing to stay and fight the spoil-heap that threatens their livelihood. Without the Mountain, the Higgins don't have the security of a home and a history that they own. And that's worth really fighting for.