How do you identify a sentence fragment

Read the excerpt from “the fall of the house of usher” by edgar allan poe. suddenly there shot along the path a wild light, and i turned to see whence a gleam so unusual could have issued; for the vast house and its shadows were alone behind me. the radiance was that of the full, setting, and bloodred moon, which now shone vividly through that once barely discernible fissure, of which i have before spoken as extending from the roof of the building, in a zigzag direction, to the base.