The Northampton Sand Formation, sometimes called the Northamptonshire Sand, is a Middle Jurassic geological formation which is placed within the Inferior Oolite Group. It was formerly worked extensively in Northamptonshire for its ironstone. The Northampton Sand Formation constitutes the lowest part of the Inferior Oolite Group and lies on the upper Lias clay. It attains a maximum thickness of up to 21 m to the north and west of Northampton where it lies in a subterranean basin. In the south, it fades out around Towcester. Northward from the edge of the basin in the upper Lias, under Northampton, it lies progressively lower beneath the Jurassic Lincolnshire limestones. A little to the north of Corby Glen it is at about 50 m from the surface. It fades out under north Lincolnshire as the strata rise towards the Market Weighton Axis. The formation to dates to the Aalenian, and predominantly consists of sandy ironstone, which when freshly exposed is greenish-grey in colour, which weathers to limonitic brown sandstone. It formed in an extensive, shallow sea on the northwestern margin of the London-Brabant Massif. Dinosaur remains are among the fossils that have been recovered from the formation, although none have yet been referred to a specific genus. A species of horseshoe crab, Mesolimulus woodwardi has been described from the formation. The formation is a signficiant emitter of radon gas.

Commercial exploitation

There is a description of the twentieth century exploitation of the Northampton Sand for iron-smelting in the Wellingborough article.