here), he wades through the ghost seam which comes as high as his thighs. (p. 548) Watching from Upstairs, Michael sees "...the stacked-up planes below." (p. 550) "...underneath the ghost-seam..." are "...the twining spark-lit fronds of coral that were living people..." (p. 551)
I think that the mortal world should comprise the world-lines not only of human beings but also of every material object and particle? Michael thinks that the angelic fight is affecting the lower planes. This would mean that, in the temporal dimension of Upstairs, the history of the mortal world was different before the angelic fight than it was during the fight.
When the Dead Dead Gang watched Parliamentarians before the Battle of Naseby, I thought that they were time traveling in the mortal world. However, the text sometimes implies that they are in the ghost-seam, e.g.: "...the ghost-seam's murk..." (p. 577) Is the ghost-seam both a plane above the mortal world and the mortal world as seen by ghosts? This is the sort of uncertainty that we have on first reading a complex text. I remember, with Watchmen, not knowing how many super-powered beings there were and not realizing that Rorschach had been in our faces from page 1.
I have only just passed the half way point of Jerusalem.