Northampton Sand Formation
(Inferior Oolite Group)
Image: Box weathering in Northampton Sand Formation (Ⓒ Crown Copyright)
* Ma is an abbreviation for million years
Sandy, ironstone, greenish grey where fresh, weathering to brown sandstone typically with a box stone structure. The common iron minerals in the fresh specimen are berthierine, siderite and limonite (goethite). The lowest part is commonly muddy and less iron-rich (ferruginous). The uppermost beds are generally more-or-less ferruginous sandstone. It can include lenses of mudstone and limestone in places, and contains a fairly abundant marine fossils of bivalves, brachiopods and ammonites.
The ‘box stone structure’ results from the redistribution of the iron oxide in the rock so that it is concentrated (dark layers) and leached from the pale layers. This occurs when the ironstone is weathered forming the mineral goethite. This weathering process pretty much destroys the fossil assemblage.
The upper boundary is a sharp, unconformable contact of sandy, berthierine-ooidal and sideritic ironstone overlying mudstones of the Whitby Mudstone Formation (Lias Group), commonly marked by a pebble bed containing phosphatic nodules and derived fossils from the underlying Whitby Mudstone.
The formation has a lower boundary marked usually by a sharp erosional, or in some places apparently transitional boundary, from sandy, berthierine-ooidal and sideritic ironstone with the Horsehay Sand Formation.
Caps many flat-topped spurs and outlying hills for example Jesters Hill at Shutford, Madmarston Hill near Tadmarton and Yarn Hill near Epwell. Distinctive orange-brown soil developed, with abundant ironstone fragments.
From 4m to 8m.
Across North Oxfordshire from the Cherwell Valley to the Chipping Norton area.
Dunston Top Pit
No Reference section in Oxfordshire
Berthierine is a dark olive green to yellowish green mineral. It is an iron-rich, aluminous, 1:1-type layer silicate belonging to the serpentine group.
Siderite is a dark to light brown (sometimes yellow/grey/white). It is iron-rich (Iron carbonate) and belongs to the calcite group of minerals.
Image 1:Berthierine mineral (© Modris Baum)
Image 2: exposure of the Northampton Sand Formation (© BGS)
Brachiopods have a very long history of life on Earth; at least 550 million years. They first appear as fossils in rocks of earliest Cambrian age and their descendants survive, albeit relatively rarely, in today’s oceans and seas. They were particularly abundant during Palaeozoic times (248–545 million years ago) and are often the most common fossils in rocks of that age.
Rocks containing iron oxides (rust).
A sedimentary rock that is composed of a substantial fraction of rounded to sub-angular fragments of older, eroded rocks.