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1 MIN QUIZ

We asked you to:

Take a look at these thin sections of rocks viewed under cross-polarised light. All of these rocks can be described as being rich in minerals from which mineral group?

The answer is:

The Olivine Group

1. Olivine-dolerite with ophitic pyroxene.

2. Olivine-teschenite with sector zoned titanium augite (TI augite pink in thin section).

3. Olivine rich Dunnite adcumulate.

Adcumulates are rocks containing ~100–93% accumulated magmatic crystals in a fine-grained groundmass.

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Rock of the Month (July)
The rocks that make Oxfordshire

Marlstone Rock Formation

A hard massive Jurassic limestone, with an abundance of fossils and iron-rich. It's blue or green centred when fresh but readily weathered to a rich brown. This formation crops out in the northern part of Oxfordshire between Banbury and Chipping Norton.

Its warm, chocolate hues characterise the local settlements in this part of the Cotswold Hills.

In some places the Marlstone Rock contains enough iron to be exploited as an ore, giving rise to much industry in the 19th & 20th centuries.

"When sawn or dressed the deep (unweathered) rock reveals green/blue veins of great beauty". Sir Henry Moore's 'Recumbent Figure' (1938), commissioned by the architect, Serge Chermayeff, used Marlstone Rock sourced from a quarry near Banbury.

Image: Recumbent Figure 1938 (Ⓒ Tate Gallery)

On the Rocks
with Alejandro

Alejandro Baeza is a great rock climbing enthusiast with many years experience.

You can see Alejandro is a good part of the way through his climb at Penmaen Head in North Wales.

The rock he is shinning up is known as the Clwyd Limestone Group. It is a limestone that was formed circa 329-347 million years ago in the Carboniferous Period.

A diverse range of limestone facies (the character of a rock expressed by its formation, composition, and fossil content) with some minor sandstone and mudstone units, and exhibits local dolomitisation (where a limestone comes into contact with magnesium-rich water and converts to dolomite).

This rock unit marks the start and growth of a carbonate platform along the northern flank of the the ancient Wales-Brabant Massif.

These rocks would have been deposited at the bottom of a shallow sea, not too far from the coastline. Occasional on-land storms would have washed muds and sandy deposits out, forming the minor sandstone and mudstone units.

The composition of the limestone is biogenic (produced by living organisms) and detrital (disintegrated-broken/eroded material). They generally comprise carbonate material eg coral and shell fragments forming beds and in some places palaeoreefs.

The location is 44 m OD.

"Penmaen Head is a relatively recent addition to the climbing on North Wales limestone and has proved to be extremely popular. The crag is made up of a number of walls that have been developed to give single-pitch sport climbs, the majority being in the 6th grade. The rock is generally good and the routes well-equipped allowing plenty of mileage to be gained in a short period. Some of the faces are covered with flowstone and these in particular are well worth stopping off to sample either when travelling past on the A55 or as an escape from the poor weather in the mountains. The crag faces just north of west getting all the afternoon and evening sun that is available. It dries quickly and doesn't suffer from any long term seepage. The Penmaenrhos Wall may give some dry climbing in light rain, and is also sheltered making it a good bet if retreating from the wet and windy mountain crags." (ukclimbing.com)

Jurassic Zone Ammonites #02 Quenstedoceras lamberti

Ammonites are common & conspicuous fossils in Mesozoic marine sedimentary rocks. Ammonites are an extinct group of cephalopods - they’re basically squids in coiled shells. The living chambered nautilus also has a squid-in-a-coiled-shell body plan, but ammonites are a different group.

The origin of the name Ammonite is from the Ancient Greek. The coiled shell shaped of an ammonite is said to be reminiscent of a ram’s horn. The ancient Egyptian god Amun (Ammon in Greek) was often depicted with a ram’s head & horns.

Pliny’s Natural History, written in the 70s A.D., refers to these fossils as Hammonis cornu (the horn of Ammon), and mentions that people living in north eastern Africa revered them as sacred. Pliny also observed that ammonites were often pyritised.

At present, the 11 Jurassic stages (representing approximately 70 million years) can be divided into 145 ammonite-based zones or subzones. Quenstedoceras Lamberti defines the Lamberti Zone at the very top of the Callovian stage circa 163.5 ± 4.0 Ma. Image ca 3.5 cm width.