Special Edition: Chalk, Snow, and Roman Numismatics (or How Geology Can Shape a People)

It’s the weekend, so why not have a new Special Edition?  (I’ll be honest, I didn’t actually write this today, I wrote it several days ago, as I’m revising practical papers this weekend).  Anyway, the title of this post is of three things that seem pretty unrelated at first, so why a special edition about them?  Well, one day, while procrastinating, I noticed something interesting, a correlation between them.  Also, it’s a little taste of home, which is nice to remember in the midst of all these dastardly exams.

First of all, we need a geological map of Britain.  The area with the box is what we’re looking at today.  Look at the arrow.  You see there is a horseshoe shape of greens and blues in that region?  Good!  Make sure you remember that shape:

The Geology of Southern England: British Geological Survey/Natural Environment Research Council

The Geology of Southern England: British Geological Survey/Natural Environment Research Council

Right, now that we’ve got the geological basis of the post laid down, I’d better get on with the snow (appalling pun intended)! (I know it’s hard to imagine snow in June, with its (occasional) warm sunny days, but give it a go).  Earlier this year, there was a lot of snow across our verdant isle, and a satellite picture was taken by those chaps over at NASA:

Satallite photo of Great Britain: NASA

Satellite photo of Great Britain: NASA

What do we see?  That same horseshoe shape.  Why is it there?  It’s fairly straightforward.  There was a large fold present originally, but now the centre part has eroded away.  As a result, there are now two series of hills that meet in the west (the North Downs – known to me as “home”, and the South Downs).  Snow settles more easily on higher ground, because it’s colder, so it stays on these hills for longer.

Now, let’s move onto where the Romans come into this. “What have they got to do with this geology?” I hear you ask.  Well, let’s have a look.  The Romans were a busy bunch, first invading properly in 43, and not withdrawing until around 410 or so.  As a result, there is a lot of Roman archæology knocking about.  A while ago, I came across a map of where Roman coins had been found over the past 20 years or so, and noticed an interesting pattern.  Yes readers, that horseshoe is back once more (highlighted below).

Locations where Roman coins were found between 1997 and 2010: Portable Antiquities Scheme

Locations where Roman coins were found between 1997 and 2010: Portable Antiquities Scheme

Not being a Roman expert, I can’t really make any solid argument as to why more coins may have been found here relative to the centre of the area.  Maybe my sister (if she’s reading this) could elaborate, as Classics is her thing.  Perhaps the Romans preferred the hills for their strategic value?  Either way, it’s an interesting example of how geology can shape a civilisation.

Shaken, Not Stirred

Time for some geological news.  On Friday morning, there was quite a large earthquake of the western coast of Kamchatka (in the Sea of Okhotsk), with a magnitude of 8.3. Luckily there have been no reports of damage or injuries.  Anyway, we are all accustomed to hearing about earthquakes with a magnitude of x, but what does this actually mean?  In essence, it can be considered as a measure of the energy released during an earthquake.

Modern earthquakes are not really measured by the Richter scale any more, as there are various issues it has with recording earthquakes with large magnitudes.  Instead, a different scale is used, which is called the moment magnitude scale (which does not have these problems).

The magnitude of the seismic moment is what is often reported in the news, but where does this number come from, and what does it mean? Essentially it is defined by the logarithm of the “seismic moment” (what this is will be explained later), with a few constants thrown in (so it is similar to the Richter scale, as that is what people are used to).  For those of you who are interested, the magnitude (Mw) is defined as being:

Mw

The seismic moment (M0) is defined by the following equation:

M0

“A” represents the area of the fault that slipped in the earthquake, with “d” being the distance it moved.  “μ” is the “rigidity modulus”, which, for an earthquake, describes how the rock changes shape when one of its faces is subjected to a shear force, while the other is subjected to frictional resistance.  You can see this for yourself.  Put your hand on a wooden table, and try to slide it forwards.  You’ll be able to feel your skin resist the movement, and see it change shape slightly (look at the tips of your fingers).  It is this resistance and shape changing that the rigidity modulus describes.

Plugging in the numbers for this particular event, the seismic moment comes out as being around 3 billion trillion Newton metres. (3×10^21 Nm).  Put another way, the amount of energy this represents is enough to provide electricity for the whole of the United Kingdom for the next 250 years (or, enough energy to make about 30,000 trillion cups of tea).  For comparison, the largest earthquake ever recorded on Earth had a magnitude of 9.5, which took place in Chile in 1960, and is equivalent to releasing 15 times as much energy as the magnitude 8.3 earthquake yesterday in Kamchatka.

The William Smith Map of 1815

So today, we got an email from the Department saying that (one of their) copies of the William Smith map (touted as the first ever geological map in the world) would be on display, so I gave LB a ring (as a IA she didn’t get the email sadly), and we went to have a look at it.  My mapping supervisor (sarcastically of course) said he’d give it a 2.ii if he was marking it, partly due to an issue with the Variscan unconformity.  Anyway, needless to say it was very awesome (although as I’m obsessed with both maps and geology, that’s hardly a surprise), and a couple of pictures are attached for your perusal. Enjoy!

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