How does Susan Stewart use imagery in The Forest?

In the fourth stanza, forest imagery begins to creep in ("black humus there") as the process of memory starts to work, although the parallel imagery of the sea seems to work against the formulation of any concrete, earthy images. In the last line of the fourth stanza, Stewart uses musical imagery ("like a light left hand descending, always on the same keys"), to suggest a pianist playing the same chord over and over again. This image of repetition implies that melody has been lost; no development is possible, which relates to the struggle to recall an experience now departed.