Sentence:  ... there came no answer, except the echo of my own voice sounding hollow and far off down in the vault. So in despair I turned back to the earth wall below the slab, and <WSD>scrabbled</WSD> at it with my fingers, till my nails were broken and the blood ran out; having all the while a sure knowledge, like a cord twisted round my head, that no effort of mine could ever dislodge the great stone. 
Meaning of the word marked with <WSD> tags:  To scrape or scratch powerfully with hands or claws.