Struggling with this - a quote/passage from a book, diary or journal (or possibly TV/DVD although it's supposed to be 'literal'). The author may have a literary or walking background or connection to Lake District, but what he/she is writing about definitely is (reference to High Street plateau).
For the views described I'd suspect writer to be describing view from Helvellyn area - maybe.
Q. Where was who stood?
"Smooth snow-slopes dipped down for a mile to the brown, shadowed valley, and beyond that the late sun lingered on the snows along the whole length of the High Street plateau. And in the distance, hanging in the sky without visible support from below, was the shining white outline of the Northern Pennines, the biggest mountain mass in England"?
Compiler may have used punctuation or redaction to foil searches.
johnT wants to know where someone (to be identified) was standing in order to see the High Street plateau, with the Northern Pennines beyond. The Northern Pennines are east of High Street so the observer is somewhere to the west of it, hence his suggestion of Helvellyn.