We first see Miroku's father's grave in episode 28 when he leaves the group to have the lingering problems with his wind tunnel fixed. In that same episode we see a flashback to the death of Miroku's father and a glimpse into Miroku's own death if he fails to kill Naraku.
Located near Mushin's shrine, Miroku's father's grave is a typical gorintou (五輪塔), or five-tier grave. Each piece in the five-story grave corresponds to one of the five elements. The square bottom story corresponds to the earth ring. Next is the spherical water ring with the triangular ring of fire on top of that. Above that is a reclining half-moon representing the wind, and on top is the gem-shaped ring of space.
The bowl-shaped depression it sits in is anything but typical. The crater is the bottom half of the spherical void that happened when Miroku's father's Wind Tunnel grew large enough to engulf him. As shown in the flashback sequence Miroku would have died as well if Mushin hadn't stopped him from rushing to his father.
You can see pretty much the same process happen in the first Inuyasha movie when that stupid moth demon what's-her-name opens up her copycat wind tunnel and gets herself sucked in. Complete with a similar crater left in the tree limb she was standing on.
Since Miroku's father left no remains when he died, the grave is purely a memorial at the center of the crater and doesn't mark or contain any part of him.
