Angle of Repose ends in the strangest and most unexpected way possible.
First, we witness the sad end of the Susan-Frank affair. Based on his research, Lyman determines that Oliver and Susan's youngest daughter, Agnes, drowns in a canal after accompanying her mother on one of her trysts with Frank. Frank commits suicide several days later, presumably out of guilt for indirectly causing the young girl's death. So, yeah—that's a bummer.
Lyman's story, on the other hand, ends rather abruptly. First, he has a really trippy dream (which we write about in more detail in our "Symbolism, Imagery, Allegory" section) that involves his ex-wife, Ellen, and a topless Shelly, and an awkward incident involving a catheter. It's a lot, we know. But, the important part is the effect this dream has on Lyman—it causes him to wonder if he should forgive Ellen and try to make some sort of amends.
In the end, Lyman finally makes the connection between his own marriage and his grandparents' marriage, saying that he hopes that he's "man enough to be a bigger man than [his] grandfather" and forgive Ellen (9.1.240). Oliver was never quite able to let go of his anger toward Susan, but Lyman hopes to do better than the old man's example.