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Although you don't have to join OGG to enjoy most of our activities (it is cheaper to participate for OGG members) we would love for you to take membership. Membership has its benefits and we would be happy to talk to you about this.

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Rock of the Month
The rocks that make Oxfordshire

Northampton Sand Formation

170.3 -174.1 Million Years Ago (Jurassic)

Inferior Oolite Group

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.

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Jurassic Zone Ammonites #01 Psiloceras planorbis

This fossilised species first appears in the very earliest rocks of the Jurassic and can be found in rocks of the same age across the world. Therefore, The International Commission on Stratigraphy (ICS) has assigned the point where the fossils can be first seen in the rock strata as the defining biological marker for the start of the Hettangian Stage of the Lower Jurassic. So, if you find this specific ammonite in a rock, you're looking at a creature that was swimming around in the seas at the beginning of the Jurassic Period around 201.3 ± 0.2 million years ago, more importantly you can reliably date the rock you are looking at - anywhere in the world. Cool eh?