Moor Mill Quarry, West is a 0.16-hectare (0.40-acre) geological Site of Special Scientific Interest in How Wood in Hertfordshire.

The local planning authority is St Albans City and District Council.

It was notified in 1992 as representing the former Moor Mill Quarry SSSI, which was lost to landfill operations.

This was overrun by a second advance of ice during the Anglian glaciation around 450,000 years ago, which diverted the Thames to its present course and initiated Colne drainage in the area.

The site is regarded by Natural England as of fundamental importance as the only one which demonstrates the diversion of the Thames from its pre-Anglian course.