When forty winters shall besiege thy brow,
And dig deep trenches in thy beauty's field,
Thy youth's proud livery so gaz'd on now,
Will be a totter'd weed of small worth held:
Then being asked, where all thy beauty lies,
The decaying aspect of time is seen in the physical appearance of wrinkles.  The  metaphor , comparing a beautiful human face with a field of trenches, is inventive and original. 
 Oddly and fortuitously, this may bring to some readers' minds the trenches of World War One, though of course Shakespeare couldn’t have predicted the future. Common to both eras is the idea of the destruction of youth and beauty. 

Sonnet 2