How is tangibility shown in Sylvia Plath's poetry?

The poem begins with complete immobility in the darkness, while the rider waits on the horse. There is then a change – the intangible blue of hills and distances come into being. The rider is "God's lioness;" she experiences the sensation of becoming one with her horse in a powerful entangling of knees and heels. The plowed field on which she rides soon splits and vanishes behind her, remaining elusive like the brown neck of her steed that she "cannot catch."