Oliver is an engineer, and his life revolves around mines. Susan is an artist and a writer, married to a man whose life revolves around mines. As such, the literary significance of mines, mining terminology, and the notion of untapped resources buried in deep and impossible-to-reach places becomes an important recurring motif.

The novel's title takes its name from a mining term, the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. Lyman finds a quote from his grandmother in which she adores the literary symbolism of this phrase but does not explain what she means. It is a mining term, demonstrating how she is incorporating the minutia of her husband’s life into her works. Lyman ponders her meaning, his explanations varying from the bluntly literal (him lying down) to the flippantly symbolic (the crumbs of dust tumbling down the side of the tragedy-infused canal). That Lyman never settles on one certain answer is significant: The importance of interpretation is clear and, in that respect, this one metaphor becomes an analogy for his entire project. Just as Lyman is trying to deduce what his grandmother meant regarding this one metaphor, he is trying to deduce how she lived her life and how he might follow in her footsteps.