Various online thesaurus sites have in their databases the word duskheap, as a synonym for e.g. midden. This looks very likely to be a typo for dustheap; no duskheap seems to be known even (no pun intended) to the O.E.D. What a beautiful error though: imagine digging on a duskheap, through the layered remnants of countless yestereves, long-spent twilghts and disused mirknings, sunsets piled on sunsets in a loaming of gloaming.