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 metres (69 ft) to the north and west of Northampton where it lies in a subterranean basin.

Northward from the edge of the basin in the upper Lias, under Northampton, it lies progressively lower beneath the Jurassic Lincolnshire limestones.

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.

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