Those mussel shells appear to persevere, despite resting atop a heap of dead life forms. They "adjust" despite constant reminders of death, just like people who must also "adjust" when confronted with similar reminders.
[…] The water drives a wedge of iron through the iron edge of the cliff; (18-20)
Even the cliff has some figurative iron armor to protect it ("iron edge"). It's tough and, even if the water is driving a "wedge of iron" through it, we get the sense that the cliff will withstand the abuse.
Repeated evidence has proved that it can live on what can not revive its youth. (37-40)
We don't know for sure what that cliff is "liv[ing] on," but we do know that there's something there that keeps it alive. So it endures with some sort of invisible strength. And people appear to do the same, whether they rely on faith, willpower, or love—even if these things can't fix us or make us young again.