At first, the speaker is puzzling over a common turn of phrase: people say a plow can plow snow. But plowing normally prepares soil for planting, and you cannot plant anything in snow. So the expression only makes sense, the speaker suggests, if it is meant bitterly—a mockery of what it feels like to have spent years trying to farm rock. That phrase “cultivated rock” turns the image hard and cold: the land itself seems to resist every effort.
The feeling is dry, skeptical, and quietly grim. Rather than complaining openly, the speaker uses a small, almost playful logical point to expose a large sense of futility. The mood is wintery and spare, with humor so dark it barely warms anything.