Ouch! I see that the two months have passed since my last email. Two common themes appear to be running through my newsletter, one of which of course being Tony Brink’s wonderful stuff, the other being an inability to get it out on a monthly basis. I will need to up my game. But as we all know there are so many distractions and challenges in life, and then we have had a very busy spell, and I had to go abroad to attend to some family matters.
Transported Soils of Mixed Origin
So without further ado let us press on with the tale of Brink. Well, his next chapter is titled Transported Soils of Mixed Origin, which is a catch-all chapter for those sands that aren’t easy classified. These sands are not wind-blown, nor are they of colluvial or alluvial origin. They have developed their properties as a result of various processes over time, which include bioturbation, redistribution and pedogenesis. In places these processes have been so pervasive that it is impossible to tell the provenance of the original soil. Bioturbation is one of the most important, with termites carrying the finer material of residual origin to the surface, burying the biogenic stone line at depths of generally less than 1 m but at times up to 6 m.
They have low in situ densities
Much like the aeolian deposits written about last time, these soils have a collapse potential. They are located in the more humid regions of South Africa where Weinert’s N Value is less than 5 and decomposition is the dominant form of weathering. They often comprise silty or clayey Sands, sandy Silts or sandy Clays, depending on the degree of weathering and the original ratio of quartz to decomposable material. They are generally intact but have low in situ densities. Their reddish or orange brown colour is due to mobilisation of iron oxides during weathering. Ferricrete is often developed as gravels or even hardpan.
No collapse occurs in soils with dry densities greater than 1600 kg/m3
Clays within the soil mass are leached out, leaving a skeletal structure of quartz grains supported by the remaining clay and iron oxides. They will consolidate normally above a critical moisture content but will collapse when loaded and the moisture content is raised. Tests show that the collapse potential index increases with decreasing dry density and degree of saturation, and that no collapse occurs in soils with dry densities greater than 1600 kg/m3. This is a wonderful tool when it comes to making an initial assessment of the collapse potential of a soil.
These soils may provide spurious heave potential results based on indicator test results and will most probably have low angles of phi which is strange for sandy material.
Solutions to the problem of collapse are relatively straight forward. These materials compact well and densification by in situ treatment or removal and recompaction often solves the problem, especially for lightly loaded structures. But again forewarned is for-armed so once again we owe a debt to Brink for pulling all of these lovely case studies together into an accessible and readily available volume.
Hutton’s Unconformity – Geology’s Holy Ground
What else has been going on here at the GeoZone? Well, I was in Scotland for too short a time, and then in England last month too. Scotland is always a homecoming and I had the privilege of scrambling down an extremely precipitous cliff to Siccar Point, on the Berwickshire coast, to stand on one of the holy spots of geology. It was here that the acknowledged father of the science, James Hutton, confirmed his theory that the world was immeasurably old.
The evidence was a pile of red sandstones, gently dipping towards the sea, lying on vertically bedded greywackes of clearly much older age. He wasn’t able to put dates to these sediments as the science hadn’t developed enough at that time. What he was able to say was that the underlying greywackes had had sufficient time to be deposited, go through the process of diagenesis, be tilted up by powerful tectonic forces, be subsequently eroded, and then have the sandstones deposited thereon, which in turn had sufficient time to turn into solid rock.
We now know that the overlying Old Red Sandstone is of Devonian age, and the underlying Llandover Greywackes date back to the Silurian, with a 65 million year gap between the two events. Geologist see these kinds of unconformities everywhere, and South Africa has more than its fair share, but it was here at Siccar Point where it was first recognised. And so I had the privilege of being able to stand on this storm -swept promontory above the North Sea, with only the mewling gulls for company, and contemplate the significance of James Hutton’s genius.
As an aside, James Hutton was an abominable writer, and it was a certain John Playfair who took his work and put it down in plain, accessible English. Playfair was in the boat that took the party to Siccar Point back in 1788, and it is his writing that gave us access to Hutton’s work. Unlike Hutton, our Tony Brink did not suffer from that handicap.
Marmaduke the Dinosaur
Holidays saw a hoard of kids traipsing through my door, fascinated by my ornery critter and the bones and shells and minerals on my display cabinet. A bit disruptive to be sure, but then again, the look on their faces was priceless. I am still giving my dinosaur colouring book away, so let me know if you would like a copy. It is in PDF form so you will have to print it out yourself.
I see that I am slightly over my allotted 800 words, but what is one to do when there is so much to say? I have omitted my trip to the Oxford University Museum of Natural History so that awaits another day.
If you found this interesting please forward on to colleagues and friends, and seeing there are no shortage of electronic copies of the dinsosaur colouring book, let all the kids know too.