Ashes to ashes, dust to dust is what they said over his father's grave and he didn't like it. He knew it was all about his father going up to god and being able to rest properly but it didn't feel right. His father hadn't died a restful death, he'd been shot down and left in the dust by men who saw him as in the way.
The reverend saying the words didn't really know his father and what dust meant to him, he just knew how things are meant to be done.
Dust was what their land was, dust that caught in Mark's throat and made him cough, dust that made his mother look drab and tired, dust that drove his father to die.