[Buddha-l] Help in translation

[dorje Pizza]

"I am reading a text of Longchenpa and I found this passage
"

These manifest in our

perception as distinct from one another, but their nature is this...complete

purity. These do not fall outside of the view of this pervasive state of

timeless perfection, that of dredging this pit of samsara and nirvana"

I did not get the meaning of the senctence 
"that of dredging this pit of samsara and nirvana"
 in the contest of the  precedent words
Is it  an idiomatic expression ? .Someone can explain me

Yours
Dorje Pizza