Sunday sand: the green, green grains of Wissant

 
The mineral glauconite, so named from the Greek glaucos, meaning, oddly enough, blue-green. Glauconite is an iron-potassium member of the phyllosilicate group of minerals, the micas, whose crystalline form appears as thin sheets  - if you insist, glauconite’s empirical formula is K_0.6Na_0.05Fe³⁺_1.3Mg_0.4Fe²⁺_0.2Al_0.3Si_3.8O₁₀(OH)₂

In a previous Sunday Sand post, I described the green grains of Groningen, and the mineral responsible for the colour – and here it is again, even greener, even bluer, this time from the coast of France. Glauconite takes on a variety of mineralogical guises, but these are typical sand-sized pellets or rounded aggregates, rolled around on the floor of an ancient tropical sea. It’s glauconite that is the pigment for many of the green sands around the world (and for some of the hues of Russian icons). Around Northern Europe, beneath the great chalk cliffs of Dover, Calais and Denmark, lies a pile of sand and clay layers, many of which are glauconite green and that tell stories of the seas along whose shores the dinosaurs frolicked.

It’s from one of these clay layers that today’s grains have come, weathered out along the coast of the Pas de Calais region, not far from the town of Wissant:

 
This clay, well-known in the UK as The Gault, formed in moderately deep seas around 100 million years ago; it’s a “stiff clay” that has often been used for brick-making; however, its major role has been in revealing the geological story of that era, for it contains a treasure trove of fossils – everything from minute foraminifera to ammonites, crustaceans, reptile bones, and sharks’ teeth. Here’s a spectacular Gault ammonite from the Fossils of the Gault Clay site, a comprehensive review of these spectacular collections:

 

The importance of the geology along the stretch of coast around Wissant was first recognised in the first half of the 19th century by the great and gloriously named French geologist,  Alcide Charles Victor Marie Dessalines d'Orbigny. D’Orbigny was a gentleman naturalist, typical of those times, who made significant contributions not only to geology and palaeontology, but also to archaeology and zoology. It was d’Orbigny who first recognised and named the wonderfully diverse, microscopic critters, the foraminifera, who have featured in a number of Sunday Sand posts. And it was at the Pas de Calais coast that d’Orbigny was the first to decipher the clays and green sands and see that they represented a key paragraph in the chapter of the earth’s history that is the Cretaceous Period. The name “Cretaceous” derives from the Latin word for “chalky” – remember the white cliffs of Dover or the vineyards of the hills around Austin, Texas or Champagne. The Cretaceous period began around 145 million years ago, and lasted 80 million years, culminating with the climax of the extinction of all those frolicking dinosaurs. The glauconitic sands and clays of Wissant mark a key stage within the Cretaceous, a time named by d’Orbigny in 1842 as the Albian, after Alba, the River Aube in France where further key exposures of these rocks occur. The Albian has, so to speak, withstood the test of time, and is today a paragraph of our planet’s life recognised internationally as marking the end of the Early or Lower Cretaceous:

Time would move on from the Albian, the seas would become shallower, and the great thicknesses of chalk would be deposited over so much of the earth during the Cenomanian. It was the green sands and the glauconitic clays that underpinned this monumental pile – and also underpinned the Channel Tunnel, as its engineers will testify.

 

[Photograph of the Wissant coast from http://www.geog.sussex.ac.uk/BAR/images/Pictures-France%202003/Blanc_Nez/Page.html. I'm grateful, as always, to Carla Lagendijk for providing the sand grains.]

 

Comments

[Walter Vogelsberg]

Interesting, as ever. But the greatest fun are visuals I am having of dinosaurs frolicking. From a distance.