Not only the bottom of that chamber but those rocks themselves will start to collect detritus in all their nooks and crannies over time. You will at some point want to clean out that chamber and it will be a nightmare, probably :-)
As a suggestion, what you could use in stead of the rubble are floating dish pads.
Since they float they are less likely to accumulate detritus. At least mine never have. Plus you can easily stick a vacuum in that chamber and vacuum up the accumulated detritus. That is unless accumulated detritus is exactly what you're wanting in that chamber. In which case, carry on :-)