Former whinstone (dolerite) and limestone (Three Yard Limestone) quarry.

From Wikipedia:
The site has national geological importance for its exposures of the Little Whin Sill; there are other exposures in the bed of the Wear near the quarry, but none shows so complete of an exposure as Greenfoot Quarry. At the western end of the quarry is a unique exposure that displays columnar jointing in the sill with limestone exposed above and below the intrusion.

SSSI