Watnall pebbles and their remarkable time travels...

on


These beautiful quartzite pebbles were collected from half an hour's "beachcombing" in the fields around Watnall. Let's consider these humble pebbles for a minute, so often overlooked and walked over. Why are they so smooth, round and colourful like beach pebbles and how did they get here? They are much older than you might imagine. Their story embraces plate tectonics, ancient mountain chains, extreme climates, early life, and ultimately, powerful river systems and glaciers on our doorstep. Read on and discover their amazing story...

Every pebble is around 250 million years old, that's four times older than the dinosaurs of T. Rex's era, and such is their durability they'll be here for many more million years to come. They were forged deep underground at immense heat and pressure as a band of quartz crystal in what is today Brittany in France when two ancient continents collided. The mountains formed in the collision pushed the quartz to the surface. Over millions of years, the mountains were eroded by monsoon rains and the broken up quartz rocks were washed northwards via a gigantic ancient river system called the Budleighensis. 
The continual rolling and scouring along the river bed smoothed and rounded them into the pebbles we see today. Some of them ended up around Watnall deposited into the river's sandy bed and were then buried by later layers of sandy deposits which eventually became sandstone. Much more recently, during the last ice age around 10,000 years ago, glaciers scoured away the surface exposing the pebble beds. It's a remarkable journey over an unfathomably long time span. 

200 million years ago -
The pebbles have come from what is now Brittany in northern France

200 million years ago - Watnall during the Triassic era?
The Budleighensis river would have looked like this.

This article in Geology Today explains the science...

"Bunter quartzites: remarkable journeys in time and space - Quartzite pebbles and cobbles, commonly known as Bunter quartzites, are widely dispersed throughout southern Britain. They can be traced back to Early Triassic pebble beds outcropping in the Wessex Basin and the English Midlands. Fossils within the quartzites confirm that most, if not all, were derived from Ordovician and Devonian terrains, over what is now the general region of the Armorican peninsula of north-west France. In Early Triassic times that area of ancient rocks formed part of a chain of young Variscan mountains which were subject to a monsoonal climate, and shed vast quantities of eroded quartzite. Ultimately, this debris was transported northwards into what is now southern Britain, by the Budleighensis river system."

Nottingham Castle - Today, you can see the same quartz pebbles still embedded in the sandstone on the cliff face of Nottingham Castle and in The Park. In fact geologists call the rock type "The Nottingham Castle Sandstone Formation." 

Quartz pebbles still embedded in the sandstone of Nottingham Castle



Quartz Colours - The rich colours come from impurities and trace elements present when the quartz was forming. For example, amethyst quartz is purple because of the presence of iron impurities, while rose quartz is pink due to the presence of titanium or manganese. Smoky quartz gets its brownish-grey colour from the presence of aluminium impurities, and citrine quartz is yellow due to the presence of iron impurities.

Quartz Veins - Quartz is a crystallised form of "sand" properly called silica or Silicon Dioxide, chemical formula SiO2. At higher temperatures and pressures, such as when continental plates collide, quartz is easily dissolved by watery fluids percolating through rock. When silica-rich solutions penetrate cooler rocks, the silica will precipitate as quartz in fissures, forming thin white seams as well as large veins which may extend over many miles. 

Their Journey Continues - So next time you are walking the fields of the English Midlands, take a moment to reflect on that shiny little pebble and its remarkable journey which will continue long after we are gone, for tens or even hundreds of millions of years into the future...

300 million years ago -
Continents collide to form the Pangea supercontinent and the Variscan mountains

 
This mountain, similar to the Variscans, is made
from quartzite and is undergoing erosion


Watnall pebbles - Some are smooth and multi-coloured,
others are rougher conglomerates. Some are almost translucent.   




Sources: 
- Geology Today - Bunter quartzites: remarkable journeys in time and space
Jonathan D. Radley, Robert A. Coram - Abstract https://onlinelibrary.wiley.com/doi/10.1111/gto.12052
- Ruffell, A., & Hounslow, M. (2006). Triassic: seasonal rivers, dusty deserts and saline lakes. In P. F. Rawson, & P. Brenchley (Eds.), The Geology of England & Wales. (pp. 295-325). Geological Society of London. https://pureadmin.qub.ac.uk/ws/files/479146/GEW-295.pdf
- The Language of Stone blog - The Nottingham Castle Formation   https://thelanguageofstone.blogspot.com/2019/08/the-nottingham-castle-formation.html

Comments

Post a Comment