That's a good exercise for me, too, so I'll give it the try. The native speakers can correct my mistakes, please :-) Let's work backwards: 茫漠 is a "to-adverb", and the subclause acts on 広大さ, so the second part parses as 茫漠 と した 広大さ は 想像 を 絶する = As for the vastness made boundless, it defies imagination. から connects two sentences of the form "[reason] kara [result]". と いう is just used as generic quotation or nominalization, it doesn't mean "they said". の is the "explanatory" no. Assuming it's about some geological formation, the first part is something like: On top of which approximately 100 layers have been accumulating, so you see, that's why ... Or in plain English, something like: The vastness created by accumulating a hundred layers on top of it is so boundless that it defies imagination. Grammar links: http://www.guidetojapanese.org/learn/grammar/compound http://www.guidetojapanese.org/learn/grammar/nounparticles http://www.guidetojapanese.org/learn/grammar/define

That's so literary. Even natives will feel difficulty. I challenged translation. その上に無慮百に及ぶ階層が積み重なっているというのだから、茫漠とした広大さは想像を絶する。 The vast expanse is beyond my imagination because even it seems fact that in addition as many as about a hundred layers are piled up. その上に - in addition / moreover / on top of that 無慮 百 に及ぶ～ - as many as about a hundred 階層 - levels / layers / ranks / classes 積み重なっている - be piled up / be stacked up というのだから - because even it seems fact that / because even it is said that 茫漠とした広大さ = vast expanse (茫漠 and 広大 are almost same meaning) 想像を絶する - beyond one's imagination I'm glad if I can serve you.