Decay may be expressed here in the form of physical deterioration, or perhaps as a breakdown in the fabric of our community or society.  In either case it's generally a metaphor for change -- something passing on for something new, or an act of nature as it reclaims our presence back into the mire.  There may be symbolism suggesting a great past, or a suggestion of newfound beauty in a place of decomposition and neglect.  America is a changing society, frequently ahead of itself, often exhibiting a disposability perhaps not in alignment with nature or its own sustainability.