Tess came to the d’Urbervilles mansion as an innocent girl; she would later leave her innocence in that mansion. Although Tess is looking at Alec, Alec is actually coming from the mansion. She is leaving the mansion (her innocence) and is running away from Alec (the man who took away her youth and purity). Hence, she is leaving behind her innocence as she subconsciously runs away from her youth. She can’t have the person that once was her, so she decided to leave it behind and just look back at it.
There’s a scene from Roman Polanski’s Tess, a scene that would later be remade in subsequent screen adaptations of Thomas Hardy’s novel; it is the one wherein Tess (Nastassja Kinski) is looking back as she walks away from the d’Urbervilles mansion.
Nastassja Kinski in the 1979’s Tess
Actually, Tess is looking back because Alec (Leigh Lawson) is in his chariot chasing after her. This picture would later be recreated in 1998’s Tess of the d’Urbervilles, a TV movie starring Justine Waddell, and later in 2008’s Tess of the d’Urbervilles, a mini-series starring Gemma Arterton. Same facial expression, (almost the) same angle, and (almost the) same shot.
Justine Waddell in 1998’s Tess of the d’Urbervilles
Gemma Arterton in the 2008 mini-series
Two of the screen adaptations would even make this particular scene their poster. BBC had this as the cover of their DVD. The Criterion Collection is known for their innovative posters; but for Tess they opted to choose this particular scene as the DVD/Blu-ray’s cover.
This picture of Tess looking back — and almost looking at the audience — perfectly summarizes one of the novel’s main themes: lost innocence and the longing for it.
DISCLAIMER: No copyright infringement intended. I don’t own or claim to own any of the photos used.