GR translation: rolling right behind

[Joseph Tracy]

>>  suddenly in the sky, miles behind his back and up the river
>> mementomori a sharp crack and a heavy explosion, rolling right behind,
>> almost like a clap of thunder. 

Does anyone else wonder why  there is a comma before rolling right behind as well as after? This is my only problem with D Morris’s interpretation which seems very sensible and obvious otherwise.  Why not just a single comma after behind? To me the punctuation makes it seem like it reaches further back in the sentence or is more inclusive than the heavy explosion. Could it refer to the “river memento mori”, which is a peculiar turn of phrase, or perhaps to the sky, or to the moment he was reaching to pick his nose? 
  And... What does "up the river mementomori" mean? Here’s an explosion in the sky and miles behind him  which locates it in space; an explosion happening  just as he was reaching to pick his nose, which makes it temporal; but also an explosion happening up a river here labeled "remember you will die”. The presence of this river seems to move the explosion outside of, or behind, normal spacetime. It’s as though he has a thread or flow of memory, (this is a memory of last Sept), that is devoted to reminding him of his mortality that goes further back than this event  and it is rolling along behind him, intruding like a clap of thunder. Like a clap of thunder but not quite; parallel with a real explosion, but more than that.  
  Giving more importance to the “river mementomori” asks me as reader if this is a shared, even universal, phenomena.  It  also amplifies the mystery of Slothrop’s past, his Calvinist influences and his varied roles: as individual making sense of his life, as object of experimentation, and as focus of study by others trying to understand his connection to rocket strikes. 


> On Apr 24, 2022, at 5:47 AM, David Morris <fqmorris at gmail.com> wrote:
> 
> First was the sharp crack, followed by the heavy explosion, which was
> “rolling right behind” it (in sequence).
> 
> On Sun, Apr 24, 2022 at 1:55 AM Mike Jing <gravitys.rainbow.cn at gmail.com>
> wrote:
> 
>> V25.27-33, P26.8-14   It was Friday evening, last September, just off work,
>> heading for the Bond Street Underground station, his mind on the weekend
>> ahead and his two Wrens, that Norma and that Marjorie, whom he must each
>> keep from learning about the other, just as he was reaching to pick his
>> nose, suddenly in the sky, miles behind his back and up the river
>> mementomori a sharp crack and a heavy explosion, rolling right behind,
>> almost like a clap of thunder. But not quite.
>> 
>> What does "rolling right behind" modify here?
>> --
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>> 
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