Forest-crypt: The crypt of a church is the basement, filled
with arched pillars that sustain the building. The cavern of the
brook, as the poet will have us imagine it, is like this subterranean
crypt, where the pillars are like trees and the groined arches like
interlacing branches, decorated with frost leaves. The poet seems to
have had in mind throughout the description the interior of the
Gothic cathedrals, as shown by the many suggestive terms used,
"groined," "crypt," "aisles," "fretwork," and "carvings."