Tuesday, November 4, 2008

Walking On A Jurassic Outcrop

I am rereading a couple of chapters from Simon Winchester's book The Map That Changed The World. It is the story of William Smith a canal digger by profession who embarked in the late 1700's and early 1800's on a pioneering and ultimately heroic effort to map the geology of England. In the chapter entitled A Jurassic Interlude I came across the following passage:

The general line of their outcrop, which extends all the way north from Dorset to the Humber in Yorkshire, some two hundred miles, is one of the great dividing lines of world geology, once seen, never forgotten. Around Bath, close to where a northbound traveler like me today, Smith two centuries before me, first came across it, it is stupendously memorable.

On the western side of the line are the timid, milquetoast clays and weakling shales of the Lias, of the Lower Jurassic; on the eastern side are the tough, thick oolitic limestones of the Middle Jurassic. On the western side the consequential scenery all is valley and marsh, river course and water meadow, lowing cattle and in high summer, a sticky, sultry heat. On the eastern side, underpinned by the limestones, everything has changed: there is upland plain and moor, high hills, high wind and flocks of sheep, and in winter fine white snows blowing on what can seem an endless and treeless expanse.

And on the very line itself, at the point where England has tipped itself up gracefully to expose the limestones at its core and to reveal the huge physical contrast between the hardness and the silky softness of the Lias clays below, is a long, high range of hills and cliffs. The line is, for the most part, an escarpment edge that rolls far to the horizon, separating vales and downlands, from high plains and uplands.

It is a wonderful piece of writing and not just because it invokes images of a bucolic England. Here is yet another example of the pervasive influence of geology and geological processes on livelihoods and human economies.

In the Jurassic depositional basins of England there was a lateral facies variation from clays being deposited to the west changing to carbonate sediments to the east. As these sediments turned into rock they acquired different physical properties. The carbonates i.e. the Jurassic oolite became hardened through the precipitation of copious amounts of calcite cement in its abundant pore spaces. That cement bound the initially loose oolite particles together and transformed the sediment into hard rock. The clays which were very fine grained were not cemented into toughness. Instead they were compressed into rock but remained relatively soft.

Over time these two different rocks types got exposed to the elements and were weathered and eroded at different rates. The clay bearing rocks were softer and formed valleys and lowlands with moist organic rich soils, the hard limestones formed hills, escarpments and highlands with poorer soils. An agricultural diary economy developed on the serene clay lowlands, while a pastoral sheep economy developed on the harsher windier limestone highlands.

In my previous post I wrote about the effect of aquifer yield in basalt provinces and their control on farmer poverty. That post was the first of a thread that I have continued here and plan to write on from time to time. Geology and livelihoods. I like making this connection. It is an under appreciated theme. People always nod in agreement when oil and gas and the mining industry is mentioned. It is not hard to make that connection between geology and economy. But facies change, diagenesis, weathering and rural economies? That usually takes a while to sink in.

1 comment:

  1. It's a very interesting topic, one which affects all of us, and I look forward to reading more of these posts.