The official translation is this:
and the few dead that the years had caused among them had to be thrown off the cliff
But I think it doesn't represent exactly the Idea in spanish:
1) The expression "ir + gerund" has a mixed mening between english Present continuous and "keep + gerund"Las deudas iban creciendo:
the debts were growing consistently/ [meanwhile something else was happening / happened]Los invitados iban llegango:
The guest were arriving (one by one) - The guest keep arriving (?) [meanhwile something else was happening / happened]Yo iba tomando fotos:
I was taking pictures meanwhile something else...
So, "that the years had caused among them" is understood as "que los años les habían causado" and not as "que les iban causando"
2) In this context, the expresion "Los años" means "the life, the destiny, over the years". It's a personification of the time, in which it causes aging, illness and death[Los años] ya me han dejado el pelo blanco:
[they] have already gotten my hair white. I don't know if in english it has the same connotation, but in spanish it's very common, and not necessarily poetic nor formal / fancy. And the verb "dejar" (to leave) is slightly different to "to cause". Because its use implies more "a conviction or a gift" than "a consequence". It collocates more natural with "the years"
3) In some cases, spanish uses impersonal in situations where english uses pasive voice. So:[Los muertos] los tenían que tirar al acantilado
: The dead bodies (corpses) had to be thrown down the cliff.
But in this case, english translation seems to speak about a finished action, but in spanish the imperfect "tenían" implies an unfinished action (Otherwise, GGB would have said "los tuvieron que tirar al acantilado"). That's why it could be closer if we say "The corpses used to be thrown down the cliff"
and: y a los muertos que les iban causando los años
: Subjecttenían que tirarlos en los acantilados
So, the final meaning according to my perception is :
And the dead that the years kept leaving (were living) among them used to be thrown down the cliff