Wednesday, 27 January 2016

Overlaps and gaps

Charles Darwin spent five years 1831-1836 in his early 20s circumnavigating the World in HMS Beagle. He was on the books as 'naturalist' and was able to go ashore whenever the ship was near enough to land and the calls of naval duty allowed. In one of his memorable excursions from Valparaiso in Chile, Darwin hired horses and took off into the hinterland towards the Andes. He was forcibly struck by finding fossil sea-shells several hundred meters above sea-level. He wouldn't have been surprised at this evidence of a restless Earth, because he had a copy of his mentor Charles Lyell's Principles of Geology. But actually seeing the evidence made him start to wrestle with the idea of [im]mutability. On the same expedition Darwin was famously bitten by a barbeiro beetle Triatoma infestans which possibly delivered a load of parasitic Trypanosoma cruzi to his bloodstream.  This infection may have developed later into Chagas' disease. some of whose symptoms the great man displayed: "spasmodic daily & nightly flatulence: occasional vomiting, preceded by shivering, hysterical crying, dying sensations; copious very pallid urine; singing of ears, rocking, treading on air; vision  focus & black dots . . ."  Other people are convinced that Darwin was a pampered neurotic who was a bit of a hypochondriac.

Mais revenons nous a nos geologiques.  One of the Principles that Lyell set out was Stratigraphy, the idea that older rocks underlie younger ones. It's not always true - sometimes the rock layers have been dumped on their sides [as R at Loughshinny just N of Dublin] through some cataclysm or even tipped upside down - but it's a rule, a principle. If you present yourself at the base of the working face of a quarry, the assumption is that the older rocks are in front of you and you'll need a ladder to chip away at younger facies. The problem for geologists is that the face of a quarry or the side of an eroded valley is only so high and the distance between top and bottom only exposes a part of the geological record. To get the full picture has required investigations all over the world piecing the whole story is a series of overlapping chunks.  The names of the geological periods give a hint at the scope: Jurassic exposures from the Jura Mountains in SE France; Devonian sandstones from the county in SW England, the Carboniferous is split into Mississippian and Pennsylvanian sections, Silurian is named from a Welsh tribe etc.  If you find that a characteristic set of fossils at the top of your quarry match those in a rock sample from the bottom of a quarry in Germany then you and your German correspondent have doubled the length of the record.  It's like the amazing history that dendro-chronologists have pieced together by comparing patterns in the width of tree-rings. A single tree lives for a few hundred years but the record goes back continuously for a few thousand.  It may, for example, get us a date on the eruption at Thera that wiped out the Minoans . . . or may not.  Come to think of it, piecing together lots of small data-strings because their ends overlap is how we assemble genomes.

So the usual problem is that the record in any one place is too short.  Surely that's not going to be a problem at the Grand Canyon, the biggest hole in the ground, which is as much as 1800m vertical from the rim to getting your feet wet in the Colorado River. The word is that the rocks exposed date from 200mya to as old as 2 billion years ago.  You might naively go ahead and do a calculation to reveal how much a million years is in metres. But you'd be wrong because there are huge gaps in the continuity of the geological record in N Arizona. I've zoomed [R] into a detail of the picture (which you should check out) of the geological periods exposed in the Canyon. It's the two pinky-grey sections which are most interesting to fossil hunters: the lower, narrower one is from 600-500mya, at the base of the phanerozoic which is also the base of the Cambrian period (another! chunk of the record named from Welsh rocks).  Phanerozoic means 'apparent/visible/evident animals". Before that there was life but it wasn't in the form of neat trilobites and ammonites that you'd be proud to have on your mantle-piece. The two lower grey bits in the chart [R] are very old but also very empty fossil-wise. The unconformities in the [pinky-grey] record arise because for 100 mya the area which is now Northern Arizona wasn't a shallow sea or a swamp but one of the many sorts of habitat which never allows fossils to get laid down.  It's just as well that we have lots of other exposed rocks in Mississippi and Pennsylvania to fill in the gaps.

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